{"id":1549,"date":"2018-04-11T22:52:29","date_gmt":"2018-04-12T02:52:29","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/chapter\/7-2-covalent-bonding\/"},"modified":"2018-06-22T23:43:13","modified_gmt":"2018-06-23T03:43:13","slug":"7-2-covalent-bonding","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/chapter\/7-2-covalent-bonding\/","title":{"raw":"9.2 Covalent Bonding","rendered":"9.2 Covalent Bonding"},"content":{"raw":"<div class=\"bcc-box bcc-highlight\">\r\n<h3>Learning Objectives<\/h3>\r\nBy the end of this section, you will be able to:\r\n<ul>\r\n \t<li>Describe the formation of covalent bonds<\/li>\r\n \t<li>Define electronegativity and assess the polarity of covalent bonds<\/li>\r\n<\/ul>\r\n<\/div>\r\n<p id=\"fs-idp18344448\">In ionic compounds, electrons are transferred between atoms of different elements to form ions. But this is not the only way that compounds can be formed. Atoms can also make chemical bonds by sharing electrons equally between each other. Such bonds are called <strong>covalent bonds<\/strong>. Covalent bonds are formed between two atoms when both have similar tendencies to attract electrons to themselves (i.e., when both atoms have identical or fairly similar ionization energies and electron affinities). For example, two hydrogen atoms bond covalently to form an H<sub>2<\/sub> molecule; each hydrogen atom in the H<sub>2<\/sub> molecule has two electrons stabilizing it, giving each atom the same number of valence electrons as the noble gas He.<\/p>\r\n<p id=\"fs-idm46739904\">Compounds that contain covalent bonds exhibit different physical properties than ionic compounds. Because the attraction between molecules, which are electrically neutral, is weaker than that between electrically charged ions, covalent compounds generally have much lower melting and boiling points than ionic compounds. In fact, many covalent compounds are liquids or gases at room temperature, and, in their solid states, they are typically much softer than ionic solids. Furthermore, whereas ionic compounds are good conductors of electricity when dissolved in water, most covalent compounds are insoluble in water; since they are electrically neutral, they are poor conductors of electricity in any state.<\/p>\r\n\r\n<section id=\"fs-idm52120656\">\r\n<h2>Formation of Covalent Bonds<\/h2>\r\n<p id=\"fs-idp19293424\">Nonmetal atoms frequently form covalent bonds with other nonmetal atoms. For example, the hydrogen molecule, H<sub>2<\/sub>, contains a covalent bond between its two hydrogen atoms. <a href=\"#CNX_Chem_07_02_Morse\" class=\"autogenerated-content\">Figure 1<\/a> illustrates why this bond is formed. Starting on the far right, we have two separate hydrogen atoms with a particular potential energy, indicated by the red line. Along the <em>x<\/em>-axis is the distance between the two atoms. As the two atoms approach each other (moving left along the <em>x<\/em>-axis), their valence orbitals (1<em>s<\/em>) begin to overlap. The single electrons on each hydrogen atom then interact with both atomic nuclei, occupying the space around both atoms. The strong attraction of each shared electron to both nuclei stabilizes the system, and the potential energy decreases as the bond distance decreases. If the atoms continue to approach each other, the positive charges in the two nuclei begin to repel each other, and the potential energy increases. The <strong>bond length<\/strong> is determined by the distance at which the lowest potential energy is achieved.<\/p>\r\n\r\n<figure id=\"CNX_Chem_07_02_Morse\"><figcaption>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"1300\"]<a href=\"https:\/\/opentextbc.ca\/chemistry\/wp-content\/uploads\/sites\/150\/2016\/05\/CNX_Chem_07_02_Morse.jpg\"><img src=\"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-content\/uploads\/sites\/387\/2018\/04\/CNX_Chem_07_02_Morse-2.jpg\" alt=\"A graph is shown with the x-axis labeled, \u201cInternuclear distance ( p m )\u201d while the y-axis is labeled, \u201cEnergy ( J ).\u201d One value, \u201c0,\u201d is labeled midway up the y-axis and two values: \u201c0\u201d at the far left and \u201c0.74\u201d to the left, are labeled on the x-axis. The point \u201c0.74\u201d is labeled, \u201cH bond H distance.\u201d A line is graphed that begins near the top of the y-axis and to the far left on the x-axis and drops steeply to a point labeled, \u201cnegative 7.24 times 10 superscript negative 19 J\u201d on the y-axis and 0.74 on the x-axis. This low point on the graph corresponds to a drawing of two spheres that overlap considerably. The line then rises to zero on the y-axis and levels out. The point where it almost reaches zero corresponds to two spheres that overlap slightly. The line at zero on the y-axis corresponds to two spheres that are far from one another.\" width=\"1300\" height=\"1275\" \/><\/a> <strong>Figure 1.<\/strong> The potential energy of two separate hydrogen atoms (right) decreases as they approach each other, and the single electrons on each atom are shared to form a covalent bond. The bond length is the internuclear distance at which the lowest potential energy is achieved.[\/caption]\r\n\r\n<\/figcaption><\/figure>\r\n<p id=\"fs-idm53665712\">It is essential to remember that energy must be added to break chemical bonds (an endothermic process), whereas forming chemical bonds releases energy (an exothermic process). In the case of H<sub>2<\/sub>, the covalent bond is very strong; a large amount of energy, 436 kJ, must be added to break the bonds in one mole of hydrogen molecules and cause the atoms to separate:<\/p>\r\n\r\n<div class=\"equation\" id=\"fs-idp80127584\" style=\"text-align: center\">$latex \\text{H}_2(g) \\longrightarrow 2\\text{H}(g) \\;\\;\\;\\;\\; \\Delta H = 436\\;\\text{kJ}$<\/div>\r\n<p id=\"fs-idm100273632\">Conversely, the same amount of energy is released when one mole of H<sub>2<\/sub> molecules forms from two moles of H atoms:<\/p>\r\n\r\n<div class=\"equation\" id=\"fs-idp43869440\" style=\"text-align: center\">$latex 2\\text{H}(g) \\longrightarrow \\text{H}_2(g) \\;\\;\\;\\;\\; \\Delta H = -436 \\;\\text{kJ}$<\/div>\r\n<\/section><section id=\"fs-idp191792320\">\r\n<h2>Pure vs. Polar Covalent Bonds<\/h2>\r\n<p id=\"fs-idp9596128\">If the atoms that form a covalent bond are identical, as in H<sub>2<\/sub>, Cl<sub>2<\/sub>, and other diatomic molecules, then the electrons in the bond must be shared equally. We refer to this as a <strong>pure covalent bond<\/strong>. Electrons shared in pure covalent bonds have an equal probability of being near each nucleus.<\/p>\r\n<p id=\"fs-idp173661472\">In the case of Cl<sub>2<\/sub>, each atom starts off with seven valence electrons, and each Cl shares one electron with the other, forming one covalent bond:<\/p>\r\n\r\n<div class=\"equation\" id=\"fs-idm2241360\" style=\"text-align: center\">$latex \\text{Cl} + \\text{Cl} \\longrightarrow \\text{Cl}_2$<\/div>\r\n<p id=\"fs-idp15241984\">The total number of electrons around each individual atom consists of six nonbonding electrons and two shared (i.e., bonding) electrons for eight total electrons, matching the number of valence electrons in the noble gas argon. Since the bonding atoms are identical, Cl<sub>2<\/sub> also features a pure covalent bond.<\/p>\r\n<p id=\"fs-idp26845616\">When the atoms linked by a covalent bond are different, the bonding electrons are shared, but no longer equally. Instead, the bonding electrons are more attracted to one atom than the other, giving rise to a shift of electron density toward that atom. This unequal distribution of electrons is known as a <strong>polar covalent bond<\/strong>, characterized by a partial positive charge on one atom and a partial negative charge on the other. The atom that attracts the electrons more strongly acquires the partial negative charge and vice versa. For example, the electrons in the H\u2013Cl bond of a hydrogen chloride molecule spend more time near the chlorine atom than near the hydrogen atom. Thus, in an HCl molecule, the chlorine atom carries a partial negative charge and the hydrogen atom has a partial positive charge. <a href=\"#CNX_Chem_07_02_HClBond\" class=\"autogenerated-content\">Figure 2<\/a> shows the distribution of electrons in the H\u2013Cl bond. Note that the shaded area around Cl is much larger than it is around H. Compare this to <a href=\"#CNX_Chem_07_02_Morse\" class=\"autogenerated-content\">Figure 1<\/a>, which shows the even distribution of electrons in the H<sub>2<\/sub> nonpolar bond.<\/p>\r\n<p id=\"fs-idp24320096\">We sometimes designate the positive and negative atoms in a polar covalent bond using a lowercase Greek letter \u201cdelta,\u201d \u03b4, with a plus sign or minus sign to indicate whether the atom has a partial positive charge (\u03b4+) or a partial negative charge (\u03b4\u2013). This symbolism is shown for the H\u2013Cl molecule in <a href=\"#CNX_Chem_07_02_HClBond\" class=\"autogenerated-content\">Figure 2<\/a>.<\/p>\r\n\r\n<figure id=\"CNX_Chem_07_02_HClBond\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"328\"]<a href=\"https:\/\/opentextbc.ca\/chemistry\/wp-content\/uploads\/sites\/150\/2016\/05\/CNX_Chem_07_02_HClBond.jpg\"><img src=\"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-content\/uploads\/sites\/387\/2018\/04\/CNX_Chem_07_02_HClBond-2.jpg\" alt=\"Two diagrams are shown and labeled \u201ca\u201d and \u201cb.\u201d Diagram a shows a small sphere labeled, \u201cH\u201d and a larger sphere labeled, \u201cC l\u201d that overlap slightly. Both spheres have a small dot in the center. Diagram b shows an H bonded to a C l with a single bond. A dipole and a positive sign are written above the H and a dipole and negative sign are written above the C l. An arrow points toward the C l with a plus sign on the end furthest from the arrow\u2019s head near the H.\" width=\"328\" height=\"225\" \/><\/a> <strong>Figure 2.<\/strong> (a) The distribution of electron density in the HCl molecule is uneven. The electron density is greater around the chlorine nucleus. The small, black dots indicate the location of the hydrogen and chlorine nuclei in the molecule. (b) Symbols \u03b4+ and \u03b4\u2013 indicate the polarity of the H\u2013Cl bond.[\/caption]<\/figure>\r\n<\/section><section id=\"fs-idp14008096\">\r\n<h2>Electronegativity<\/h2>\r\n<p id=\"fs-idm42504736\">Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called <strong>electronegativity<\/strong>. Electronegativity is a measure of the tendency of an atom to attract electrons (or electron density) towards itself. It determines how the shared electrons are distributed between the two atoms in a bond. The more strongly an atom attracts the electrons in its bonds, the larger its electronegativity. Electrons in a polar covalent bond are shifted toward the more electronegative atom; thus, the more electronegative atom is the one with the partial negative charge. The greater the difference in electronegativity, the more polarized the electron distribution and the larger the partial charges of the atoms.<\/p>\r\n<p id=\"fs-idm49461456\"><a href=\"#CNX_Chem_07_02_ENTable\" class=\"autogenerated-content\">Figure 3<\/a> shows the electronegativity values of the elements as proposed by one of the most famous chemists of the twentieth century: Linus Pauling (<a href=\"#CNX_Chem_07_02_Pauling\" class=\"autogenerated-content\">Figure 4<\/a>). In general, electronegativity increases from left to right across a period in the periodic table and decreases down a group. Thus, the nonmetals, which lie in the upper right, tend to have the highest electronegativities, with fluorine the most electronegative element of all (EN = 4.0). Metals tend to be less electronegative elements, and the group 1 metals have the lowest electronegativities. Note that noble gases are excluded from this figure because these atoms usually do not share electrons with others atoms since they have a full valence shell. (While noble gas compounds such as XeO<sub>2<\/sub> do exist, they can only be formed under extreme conditions, and thus they do not fit neatly into the general model of electronegativity.)<\/p>\r\n\r\n<figure id=\"CNX_Chem_07_02_ENTable\"><figcaption>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"1300\"]<a href=\"https:\/\/opentextbc.ca\/chemistry\/wp-content\/uploads\/sites\/150\/2016\/05\/CNX_Chem_07_02_ENTable.jpg\"><img src=\"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-content\/uploads\/sites\/387\/2018\/04\/CNX_Chem_07_02_ENTable-2.jpg\" alt=\"Part of the periodic table is shown. A downward-facing arrow is drawn to the left of the table and labeled, \u201cDecreasing electronegativity,\u201d while a right-facing arrow is drawn above the table and labeled \u201cIncreasing electronegativity.\u201d The electronegativity for almost all the elements is given.\" width=\"1300\" height=\"557\" \/><\/a> <strong>Figure 3.<\/strong> The electronegativity values derived by Pauling follow predictable periodic trends with the higher electronegativities toward the upper right of the periodic table.[\/caption]\r\n\r\n<\/figcaption><\/figure>\r\n<section id=\"fs-idm115216\">\r\n<h2>Electronegativity versus Electron Affinity<\/h2>\r\n<p id=\"fs-idp19127744\">We must be careful not to confuse electronegativity and electron affinity. The electron affinity of an element is a measurable physical quantity, namely, the energy released or absorbed when an isolated gas-phase atom acquires an electron, measured in kJ\/mol. Electronegativity, on the other hand, describes how tightly an atom attracts electrons in a bond. It is a dimensionless quantity that is calculated, not measured. Pauling derived the first electronegativity values by comparing the amounts of energy required to break different types of bonds. He chose an arbitrary relative scale ranging from 0 to 4.<\/p>\r\n\r\n<div id=\"fs-idm39789712\" class=\"textbox shaded\">\r\n<h3 class=\"title\">Linus Pauling<\/h3>\r\n<p id=\"fs-idp175623792\">Linus <strong class=\"no-emphasis\">Pauling<\/strong>, shown in <a href=\"#CNX_Chem_07_02_Pauling\" class=\"autogenerated-content\">Figure 4<\/a>, is the only person to have received two unshared (individual) Nobel Prizes: one for chemistry in 1954 for his work on the nature of chemical bonds and one for peace in 1962 for his opposition to weapons of mass destruction. He developed many of the theories and concepts that are foundational to our current understanding of chemistry, including electronegativity and resonance structures.<\/p>\r\n\r\n<figure id=\"CNX_Chem_07_02_Pauling\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"325\"]<a href=\"https:\/\/opentextbc.ca\/chemistry\/wp-content\/uploads\/sites\/150\/2016\/05\/CNX_Chem_07_02_Pauling.jpg\"><img src=\"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-content\/uploads\/sites\/387\/2018\/04\/CNX_Chem_07_02_Pauling-2.jpg\" alt=\"A photograph of Linus Pauling is shown.\" width=\"325\" height=\"394\" \/><\/a> <strong>Figure 4.<\/strong> Linus Pauling (1901\u20131994) made many important contributions to the field of chemistry. He was also a prominent activist, publicizing issues related to health and nuclear weapons.[\/caption]<\/figure>\r\n<p id=\"fs-idp80917280\">Pauling also contributed to many other fields besides chemistry. His research on sickle cell anemia revealed the cause of the disease\u2014the presence of a genetically inherited abnormal protein in the blood\u2014and paved the way for the field of molecular genetics. His work was also pivotal in curbing the testing of nuclear weapons; he proved that radioactive fallout from nuclear testing posed a public health risk.<\/p>\r\n\r\n<\/div>\r\n<\/section><section id=\"fs-idp9576080\">\r\n<h2>Electronegativity and Bond Type<\/h2>\r\n<p id=\"fs-idp186399536\">The absolute value of the difference in electronegativity (\u0394EN) of two bonded atoms provides a rough measure of the polarity to be expected in the bond and, thus, the bond type. When the difference is very small or zero, the bond is covalent and nonpolar. When it is large, the bond is polar covalent or ionic. The absolute values of the electronegativity differences between the atoms in the bonds H\u2013H, H\u2013Cl, and Na\u2013Cl are 0 (nonpolar), 0.9 (polar covalent), and 2.1 (ionic), respectively. The degree to which electrons are shared between atoms varies from completely equal (pure covalent bonding) to not at all (ionic bonding). <a href=\"#CNX_Chem_07_02_DeltaEN\" class=\"autogenerated-content\">Figure 5<\/a> shows the relationship between electronegativity difference and bond type.<\/p>\r\n\r\n<figure id=\"CNX_Chem_07_02_DeltaEN\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"1300\"]<a href=\"https:\/\/opentextbc.ca\/chemistry\/wp-content\/uploads\/sites\/150\/2016\/05\/CNX_Chem_07_02_DeltaEN.jpg\"><img src=\"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-content\/uploads\/sites\/387\/2018\/04\/CNX_Chem_07_02_DeltaEN-2.jpg\" alt=\"Two flow charts and table are shown. The first flow chart is labeled, \u201cElectronegativity difference between bonding atoms.\u201d Below this label are three rounded text bubbles, connected by a downward-facing arrow, labeled, \u201cZero,\u201d \u201cIntermediate,\u201d and \u201cLarge,\u201d respectively. The second flow chart is labeled, \u201cBond type.\u201d Below this label are three rounded text bubbles, connected by a downward-facing arrow, labeled, \u201cPure covalent,\u201d \u201cPolar covalent,\u201d and \u201cIonic,\u201d respectively. A double ended arrow is written vertically to the right of the flow charts and labeled, \u201cCovalent character decreases; ionic character increases.\u201d The table is made up of two columns and four rows. The header line is labeled \u201cBond type\u201d and \u201cElectronegativity difference.\u201d The left column contains the phrases \u201cPure covalent,\u201d \u201cPolar covalent,\u201d and \u201cIonic,\u201d while the right column contains the values \u201cless than 0.4,\u201d \u201cbetween 0.4 and 1.8,\u201d and \u201cgreater than 1.8.\u201d\" width=\"1300\" height=\"468\" \/><\/a> <strong>Figure 5.<\/strong> As the electronegativity difference increases between two atoms, the bond becomes more ionic.[\/caption]<\/figure>\r\n<p id=\"fs-idm32214656\">A rough approximation of the electronegativity differences associated with covalent, polar covalent, and ionic bonds is shown in <a href=\"#CNX_Chem_07_02_DeltaEN\" class=\"autogenerated-content\">Figure 5<\/a>. This table is just a general guide, however, with many exceptions. For example, the H and F atoms in HF have an electronegativity difference of 1.9, and the N and H atoms in NH<sub>3<\/sub> a difference of 0.9, yet both of these compounds form bonds that are considered polar covalent. Likewise, the Na and Cl atoms in NaCl have an electronegativity difference of 2.1, and the Mn and I atoms in MnI<sub>2<\/sub> have a difference of 1.0, yet both of these substances form ionic compounds.<\/p>\r\n<p id=\"fs-idm28373728\">The best guide to the covalent or ionic character of a bond is to consider the types of atoms involved and their relative positions in the periodic table. Bonds between two nonmetals are generally covalent; bonding between a metal and a nonmetal is often ionic.<\/p>\r\n<p id=\"fs-idm38253568\">Some compounds contain both covalent and ionic bonds. The atoms in polyatomic ions, such as OH<sup>\u2013<\/sup>, NO<sub>3<\/sub><sup>\u2212<\/sup>, and NH<sub>4<\/sub><sup>+<\/sup>, are held together by polar covalent bonds. However, these polyatomic ions form ionic compounds by combining with ions of opposite charge. For example, potassium nitrate, KNO<sub>3<\/sub>, contains the K<sup>+<\/sup> cation and the polyatomic NO<sub>3<\/sub><sup>\u2212<\/sup> anion. Thus, bonding in potassium nitrate is ionic, resulting from the electrostatic attraction between the ions K<sup>+<\/sup> and NO<sub>3<\/sub><sup>\u2212<\/sup>, as well as covalent between the nitrogen and oxygen atoms in NO<sub>3<\/sub><sup>\u2212<\/sup>.<\/p>\r\n\r\n<div class=\"textbox shaded\" id=\"fs-idm8119280\">\r\n<h3>Example 1<\/h3>\r\n<p id=\"fs-idp117774608\">Bond polarities play an important role in determining the structure of proteins. Using the electronegativity values in <a href=\"#CNX_Chem_07_02_ENTable\" class=\"autogenerated-content\">Figure 3<\/a>, arrange the following covalent bonds\u2014all commonly found in amino acids\u2014in order of increasing polarity. Then designate the positive and negative atoms using the symbols \u03b4+ and \u03b4\u2013:<\/p>\r\n<p id=\"fs-idm8126928\">C\u2013H, C\u2013N, C\u2013O, N\u2013H, O\u2013H, S\u2013H<\/p>\r\n&nbsp;\r\n<p id=\"fs-idm21713856\"><strong>Solution<\/strong>\r\nThe polarity of these bonds increases as the absolute value of the electronegativity difference increases. The atom with the \u03b4\u2013 designation is the more electronegative of the two. <a href=\"#fs-idm2614240\" class=\"autogenerated-content\">Table 1<\/a> shows these bonds in order of increasing polarity.<\/p>\r\n\r\n<table id=\"fs-idm2614240\" class=\"span-all\" summary=\"This table has three columns and seven rows. The first row is a header row and it labels each column. The first column header is, \u201cBond,\u201d the second is, \u201ccapital delta E N,\u201d and the third is, \u201cPolarity.\u201d Under the \u201cBond\u201d column are the following: C bonds to H with a single bond; S bonds to H with a single bond; C bonds to N with a single bond; N bonds to H with a single bond; C bonds to O with a single bond; and O bonds to H with a single bond. Under the \u201ccapital delta E N\u201d columna are the values: 0.4; 0.4; 0.5; 0.9; 1.0; and 1.4. Under the \u201cPolarity\u201d column are the follwoing: C bonds to H with a single bond, there is a lowercase delta negative sign above the C and a lowercase delta positive sign over H; S bonds to H with a single bond, there is a lowercase delta negative sign over the S and a lowercase delta positive sign over the H; C bonds to N with a single bond, there is a lowercase delta positive sign over the C and a lowercase delta negative sign over the N; N bonds to H with a single bond, there is a lowercase delta negative sign over the N and a lowercase delta positive sign over the H; C bonds to O with a single bond, there is a lowercase delta positive sign over C and a lowercase delta negative sign over the O; and O bonds to H with a single bond, there is a lowercase delta negative sign over the O and a lowercase delta positive sign over the H.\">\r\n<thead>\r\n<tr valign=\"top\">\r\n<th>Bond<\/th>\r\n<th>\u0394EN<\/th>\r\n<th>Polarity<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr valign=\"top\">\r\n<td>C\u2013H<\/td>\r\n<td>0.4<\/td>\r\n<td>$latex \\overset{\\delta -}{\\text{C}} - \\overset{\\delta +}{\\text{H}}$<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>S\u2013H<\/td>\r\n<td>0.4<\/td>\r\n<td>$latex \\overset{\\delta -}{\\text{S}} - \\overset{\\delta +}{\\text{H}}$<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>C\u2013N<\/td>\r\n<td>0.5<\/td>\r\n<td>$latex \\overset{\\delta +}{\\text{C}} - \\overset{\\delta -}{\\text{N}}$<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>N\u2013H<\/td>\r\n<td>0.9<\/td>\r\n<td>$latex \\overset{\\delta -}{\\text{N}} - \\overset{\\delta +}{\\text{H}}$<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>C\u2013O<\/td>\r\n<td>1.0<\/td>\r\n<td>$latex \\overset{\\delta +}{\\text{C}} - \\overset{\\delta -}{\\text{O}}$<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>O\u2013H<\/td>\r\n<td>1.4<\/td>\r\n<td>$latex \\overset{\\delta -}{\\text{O}} - \\overset{\\delta +}{\\text{H}}$<\/td>\r\n<\/tr>\r\n<tr>\r\n<td colspan=\"3\"><strong>Table 1.<\/strong> Bond Polarity and Electronegativity Difference<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n&nbsp;\r\n<p id=\"fs-idp64846816\"><em><strong>Test Yourself<\/strong><\/em>\r\nSilicones are polymeric compounds containing, among others, the following types of covalent bonds: Si\u2013O, Si\u2013C, C\u2013H, and C\u2013C. Using the electronegativity values in <a href=\"#CNX_Chem_07_02_ENTable\" class=\"autogenerated-content\">Figure 3<\/a>, arrange the bonds in order of increasing polarity and designate the positive and negative atoms using the symbols \u03b4+ and \u03b4\u2013.<\/p>\r\n&nbsp;\r\n\r\n<em><strong>Answer<\/strong><\/em>\r\n<table id=\"fs-idm25109184\" class=\"medium unnumbered\" summary=\"This table has three columns and five rows. The first row is a header row and it labels each column. The first column header is, \u201cBond,\u201d the second column header is, \u201cElectronegativity Difference,\u201d and the third column header is, \u201cPolarity.\u201d Under the column \u201cBond\u201d are the following: C bonds to C with a single bond; C bonds to H with a single bond; S i bonds to C with a single bond; and S i bonds to O with a single bond. Under the column \u201cElectronegativity Difference\u201d are the values: 0.0; 0.4; 0.7; and 1.7. Under the column \u201cPolarity\u201d are the following: nonpolar, C bonds to H with a single bond, there is a lowercase delta negative sign over C and a lowercase delta positive sign over H; S i bonds to C with a single bond, there is a lowercase delta positive sign over S i and a lowercase delta negative sign over C; and S i bonds to O with a single bond, there is a lowercase delta positive sign over S i and a lowercase delta negative sign over O.\">\r\n<thead>\r\n<tr valign=\"top\">\r\n<th>Bond<\/th>\r\n<th>Electronegativity Difference<\/th>\r\n<th>Polarity<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr valign=\"top\">\r\n<td>C\u2013C<\/td>\r\n<td>0.0<\/td>\r\n<td>nonpolar<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>C\u2013H<\/td>\r\n<td>0.4<\/td>\r\n<td>$latex \\overset{\\delta -}{\\text{C}} - \\overset{\\delta +}{\\text{H}}$<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Si\u2013C<\/td>\r\n<td>0.7<\/td>\r\n<td>$latex \\overset{\\delta +}{\\text{Si}} - \\overset{\\delta -}{\\text{C}}$<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Si\u2013O<\/td>\r\n<td>1.7<\/td>\r\n<td>$latex \\overset{\\delta +}{\\text{Si}} - \\overset{\\delta -}{\\text{O}}$<\/td>\r\n<\/tr>\r\n<tr>\r\n<td colspan=\"3\"><strong>Table 2.<\/strong><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<\/section><\/section><section id=\"fs-idm4705056\" class=\"summary\">\r\n<h2>Key Concepts and Summary<\/h2>\r\n<p id=\"fs-idp86079584\">Covalent bonds form when electrons are shared between atoms and are attracted by the nuclei of both atoms. In pure covalent bonds, the electrons are shared equally. In polar covalent bonds, the electrons are shared unequally, as one atom exerts a stronger force of attraction on the electrons than the other. The ability of an atom to attract a pair of electrons in a chemical bond is called its electronegativity. The difference in electronegativity between two atoms determines how polar a bond will be. In a diatomic molecule with two identical atoms, there is no difference in electronegativity, so the bond is nonpolar or pure covalent. When the electronegativity difference is very large, as is the case between metals and nonmetals, the bonding is characterized as ionic.<\/p>\r\n\r\n<\/section><section id=\"fs-idp134316112\" class=\"exercises\">\r\n<div class=\"bcc-box bcc-info\">\r\n<h3>Exercises<\/h3>\r\n1. Why is it incorrect to speak of a molecule of solid NaCl?\r\n\r\n2. Predict which of the following compounds are ionic and which are covalent, based on the location of their constituent atoms in the periodic table:\r\n<p id=\"fs-idp137731248\">a) Cl<sub>2<\/sub>CO \u00a0 \u00a0 \u00a0 \u00a0\u00a0b) MnO \u00a0 \u00a0 \u00a0 \u00a0\u00a0c) NCl<sub>3 \u00a0 \u00a0 \u00a0 \u00a0\u00a0<\/sub>d) CoBr<sub>2 \u00a0 \u00a0 \u00a0 \u00a0\u00a0<\/sub>e) K<sub>2<\/sub>S \u00a0 \u00a0 \u00a0 \u00a0\u00a0f) CO<\/p>\r\n<p id=\"fs-idp70623968\">g) CaF<sub>2 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/sub>h) HI \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0i) CaO \u00a0 \u00a0 \u00a0 \u00a0\u00a0j) IBr \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0k) CO<sub>2<\/sub><\/p>\r\n3. From its position in the periodic table, determine which atom in each pair is more electronegative:\r\n<p id=\"fs-idm12153568\">a) Br or Cl \u00a0 \u00a0 \u00a0 \u00a0\u00a0b) N or O \u00a0 \u00a0 \u00a0 \u00a0\u00a0c) S or O \u00a0 \u00a0 \u00a0 \u00a0\u00a0d) P or S<\/p>\r\n<p id=\"fs-idp165919328\">e) Si or N \u00a0 \u00a0 \u00a0 \u00a0 \u00a0\u00a0f) Ba or P \u00a0 \u00a0 \u00a0 \u00a0\u00a0g) N or K<\/p>\r\n4. From their positions in the periodic table, arrange the atoms in each of the following series in order of increasing electronegativity:\r\n<p id=\"fs-idp63429168\">a) C, F, H, N, O \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0b) Br, Cl, F, H, I \u00a0 \u00a0 \u00a0 \u00a0 \u00a0c) F, H, O, P, S<\/p>\r\n<p id=\"fs-idp288482688\">d) Al, H, Na, O, P \u00a0 \u00a0 \u00a0 \u00a0\u00a0e) Ba, H, N, O, As<\/p>\r\n5. Which atoms can bond to sulfur so as to produce a positive partial charge on the sulfur atom?\r\n\r\n6. Which is the most polar bond?\r\n<p id=\"fs-idm10175808\">a) C\u2013C \u00a0 \u00a0 \u00a0 \u00a0\u00a0b) C\u2013H \u00a0 \u00a0 \u00a0 \u00a0\u00a0c) N\u2013H \u00a0 \u00a0 \u00a0 \u00a0\u00a0d) O\u2013H \u00a0 \u00a0 \u00a0 \u00a0\u00a0e) Se\u2013H<\/p>\r\n7. Identify the more polar bond in each of the following pairs of bonds:\r\n<p id=\"fs-idp20525952\">a) HF or HCl \u00a0 \u00a0 \u00a0 \u00a0\u00a0b) NO or CO \u00a0 \u00a0 \u00a0 \u00a0\u00a0c) SH or OH \u00a0 \u00a0 \u00a0 \u00a0\u00a0d) PCl or SCl<\/p>\r\n<p id=\"fs-idp183486032\">e) CH or NH \u00a0 \u00a0 \u00a0 \u00a0\u00a0f) SO or PO \u00a0 \u00a0 \u00a0 \u00a0 \u00a0\u00a0g) CN or NN<\/p>\r\n&nbsp;\r\n<p id=\"fs-idp58442352\"><strong>Answers<\/strong><\/p>\r\n<p id=\"fs-idm21247904\">1. NaCl consists of discrete ions arranged in a crystal lattice, not covalently bonded molecules.<\/p>\r\n<p id=\"fs-idm92647632\">2. ionic: b), d), e), g), and i); \u00a0 \u00a0 \u00a0covalent: a), c), f), h), j), and \u00a0k)<\/p>\r\n<p id=\"fs-idm10691120\">3. a) Cl \u00a0 \u00a0 \u00a0 b) O \u00a0 \u00a0 \u00a0 \u00a0c) O \u00a0 \u00a0 \u00a0 d) S \u00a0 \u00a0 \u00a0 e) N \u00a0 \u00a0 \u00a0 \u00a0 f) P \u00a0 \u00a0 \u00a0 g) N<\/p>\r\n<p id=\"fs-idm60378112\">4. a) H, C, N, O, F \u00a0 \u00a0 \u00a0 b) H, I, Br, Cl, F \u00a0 \u00a0 \u00a0 c) H, P, S, O, F \u00a0 \u00a0 \u00a0 d) Na, Al, H, P, O \u00a0 \u00a0 \u00a0 e) Ba, H, As, N, O<\/p>\r\n<p id=\"fs-idm121823728\">5. N, O, F, and Cl<\/p>\r\n<p id=\"fs-idm3310160\">6. \u00a0O-H<\/p>\r\n7. a) HF \u00a0 \u00a0 \u00a0 b) CO \u00a0 \u00a0 \u00a0 c) OH \u00a0 \u00a0 \u00a0 d) PCl \u00a0 \u00a0 \u00a0 e) NH \u00a0 \u00a0 \u00a0 f) PO \u00a0 \u00a0 \u00a0 \u00a0g) CN\r\n\r\n<\/div>\r\n<\/section>\r\n<div>\r\n<h2>Glossary<\/h2>\r\n<strong>bond length:\u00a0<\/strong>distance between the nuclei of two bonded atoms at which the lowest potential energy is achieved\r\n\r\n<strong>covalent bond:\u00a0<\/strong>bond formed when electrons are shared between atoms\r\n\r\n<strong>electronegativity:\u00a0<\/strong>tendency of an atom to attract electrons in a bond to itself\r\n\r\n<strong>polar covalent bond:\u00a0<\/strong>covalent bond between atoms of different electronegativities; a covalent bond with a positive end and a negative end\r\n\r\n<strong>pure covalent bond:\u00a0<\/strong>(also, nonpolar covalent bond) covalent bond between atoms of identical electronegativities\r\n\r\n<\/div>","rendered":"<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<ul>\n<li>Describe the formation of covalent bonds<\/li>\n<li>Define electronegativity and assess the polarity of covalent bonds<\/li>\n<\/ul>\n<\/div>\n<p id=\"fs-idp18344448\">In ionic compounds, electrons are transferred between atoms of different elements to form ions. But this is not the only way that compounds can be formed. Atoms can also make chemical bonds by sharing electrons equally between each other. Such bonds are called <strong>covalent bonds<\/strong>. Covalent bonds are formed between two atoms when both have similar tendencies to attract electrons to themselves (i.e., when both atoms have identical or fairly similar ionization energies and electron affinities). For example, two hydrogen atoms bond covalently to form an H<sub>2<\/sub> molecule; each hydrogen atom in the H<sub>2<\/sub> molecule has two electrons stabilizing it, giving each atom the same number of valence electrons as the noble gas He.<\/p>\n<p id=\"fs-idm46739904\">Compounds that contain covalent bonds exhibit different physical properties than ionic compounds. Because the attraction between molecules, which are electrically neutral, is weaker than that between electrically charged ions, covalent compounds generally have much lower melting and boiling points than ionic compounds. In fact, many covalent compounds are liquids or gases at room temperature, and, in their solid states, they are typically much softer than ionic solids. Furthermore, whereas ionic compounds are good conductors of electricity when dissolved in water, most covalent compounds are insoluble in water; since they are electrically neutral, they are poor conductors of electricity in any state.<\/p>\n<section id=\"fs-idm52120656\">\n<h2>Formation of Covalent Bonds<\/h2>\n<p id=\"fs-idp19293424\">Nonmetal atoms frequently form covalent bonds with other nonmetal atoms. For example, the hydrogen molecule, H<sub>2<\/sub>, contains a covalent bond between its two hydrogen atoms. <a href=\"#CNX_Chem_07_02_Morse\" class=\"autogenerated-content\">Figure 1<\/a> illustrates why this bond is formed. Starting on the far right, we have two separate hydrogen atoms with a particular potential energy, indicated by the red line. Along the <em>x<\/em>-axis is the distance between the two atoms. As the two atoms approach each other (moving left along the <em>x<\/em>-axis), their valence orbitals (1<em>s<\/em>) begin to overlap. The single electrons on each hydrogen atom then interact with both atomic nuclei, occupying the space around both atoms. The strong attraction of each shared electron to both nuclei stabilizes the system, and the potential energy decreases as the bond distance decreases. If the atoms continue to approach each other, the positive charges in the two nuclei begin to repel each other, and the potential energy increases. The <strong>bond length<\/strong> is determined by the distance at which the lowest potential energy is achieved.<\/p>\n<figure id=\"CNX_Chem_07_02_Morse\"><figcaption>\n<figure style=\"width: 1300px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/chemistry\/wp-content\/uploads\/sites\/150\/2016\/05\/CNX_Chem_07_02_Morse.jpg\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-content\/uploads\/sites\/387\/2018\/04\/CNX_Chem_07_02_Morse-2.jpg\" alt=\"A graph is shown with the x-axis labeled, \u201cInternuclear distance ( p m )\u201d while the y-axis is labeled, \u201cEnergy ( J ).\u201d One value, \u201c0,\u201d is labeled midway up the y-axis and two values: \u201c0\u201d at the far left and \u201c0.74\u201d to the left, are labeled on the x-axis. The point \u201c0.74\u201d is labeled, \u201cH bond H distance.\u201d A line is graphed that begins near the top of the y-axis and to the far left on the x-axis and drops steeply to a point labeled, \u201cnegative 7.24 times 10 superscript negative 19 J\u201d on the y-axis and 0.74 on the x-axis. This low point on the graph corresponds to a drawing of two spheres that overlap considerably. The line then rises to zero on the y-axis and levels out. The point where it almost reaches zero corresponds to two spheres that overlap slightly. The line at zero on the y-axis corresponds to two spheres that are far from one another.\" width=\"1300\" height=\"1275\" \/><\/a><figcaption class=\"wp-caption-text\"><strong>Figure 1.<\/strong> The potential energy of two separate hydrogen atoms (right) decreases as they approach each other, and the single electrons on each atom are shared to form a covalent bond. The bond length is the internuclear distance at which the lowest potential energy is achieved.<\/figcaption><\/figure>\n<\/figcaption><\/figure>\n<p id=\"fs-idm53665712\">It is essential to remember that energy must be added to break chemical bonds (an endothermic process), whereas forming chemical bonds releases energy (an exothermic process). In the case of H<sub>2<\/sub>, the covalent bond is very strong; a large amount of energy, 436 kJ, must be added to break the bonds in one mole of hydrogen molecules and cause the atoms to separate:<\/p>\n<div class=\"equation\" id=\"fs-idp80127584\" style=\"text-align: center\">[latex]\\text{H}_2(g) \\longrightarrow 2\\text{H}(g) \\;\\;\\;\\;\\; \\Delta H = 436\\;\\text{kJ}[\/latex]<\/div>\n<p id=\"fs-idm100273632\">Conversely, the same amount of energy is released when one mole of H<sub>2<\/sub> molecules forms from two moles of H atoms:<\/p>\n<div class=\"equation\" id=\"fs-idp43869440\" style=\"text-align: center\">[latex]2\\text{H}(g) \\longrightarrow \\text{H}_2(g) \\;\\;\\;\\;\\; \\Delta H = -436 \\;\\text{kJ}[\/latex]<\/div>\n<\/section>\n<section id=\"fs-idp191792320\">\n<h2>Pure vs. Polar Covalent Bonds<\/h2>\n<p id=\"fs-idp9596128\">If the atoms that form a covalent bond are identical, as in H<sub>2<\/sub>, Cl<sub>2<\/sub>, and other diatomic molecules, then the electrons in the bond must be shared equally. We refer to this as a <strong>pure covalent bond<\/strong>. Electrons shared in pure covalent bonds have an equal probability of being near each nucleus.<\/p>\n<p id=\"fs-idp173661472\">In the case of Cl<sub>2<\/sub>, each atom starts off with seven valence electrons, and each Cl shares one electron with the other, forming one covalent bond:<\/p>\n<div class=\"equation\" id=\"fs-idm2241360\" style=\"text-align: center\">[latex]\\text{Cl} + \\text{Cl} \\longrightarrow \\text{Cl}_2[\/latex]<\/div>\n<p id=\"fs-idp15241984\">The total number of electrons around each individual atom consists of six nonbonding electrons and two shared (i.e., bonding) electrons for eight total electrons, matching the number of valence electrons in the noble gas argon. Since the bonding atoms are identical, Cl<sub>2<\/sub> also features a pure covalent bond.<\/p>\n<p id=\"fs-idp26845616\">When the atoms linked by a covalent bond are different, the bonding electrons are shared, but no longer equally. Instead, the bonding electrons are more attracted to one atom than the other, giving rise to a shift of electron density toward that atom. This unequal distribution of electrons is known as a <strong>polar covalent bond<\/strong>, characterized by a partial positive charge on one atom and a partial negative charge on the other. The atom that attracts the electrons more strongly acquires the partial negative charge and vice versa. For example, the electrons in the H\u2013Cl bond of a hydrogen chloride molecule spend more time near the chlorine atom than near the hydrogen atom. Thus, in an HCl molecule, the chlorine atom carries a partial negative charge and the hydrogen atom has a partial positive charge. <a href=\"#CNX_Chem_07_02_HClBond\" class=\"autogenerated-content\">Figure 2<\/a> shows the distribution of electrons in the H\u2013Cl bond. Note that the shaded area around Cl is much larger than it is around H. Compare this to <a href=\"#CNX_Chem_07_02_Morse\" class=\"autogenerated-content\">Figure 1<\/a>, which shows the even distribution of electrons in the H<sub>2<\/sub> nonpolar bond.<\/p>\n<p id=\"fs-idp24320096\">We sometimes designate the positive and negative atoms in a polar covalent bond using a lowercase Greek letter \u201cdelta,\u201d \u03b4, with a plus sign or minus sign to indicate whether the atom has a partial positive charge (\u03b4+) or a partial negative charge (\u03b4\u2013). This symbolism is shown for the H\u2013Cl molecule in <a href=\"#CNX_Chem_07_02_HClBond\" class=\"autogenerated-content\">Figure 2<\/a>.<\/p>\n<figure id=\"CNX_Chem_07_02_HClBond\">\n<figure style=\"width: 328px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/chemistry\/wp-content\/uploads\/sites\/150\/2016\/05\/CNX_Chem_07_02_HClBond.jpg\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-content\/uploads\/sites\/387\/2018\/04\/CNX_Chem_07_02_HClBond-2.jpg\" alt=\"Two diagrams are shown and labeled \u201ca\u201d and \u201cb.\u201d Diagram a shows a small sphere labeled, \u201cH\u201d and a larger sphere labeled, \u201cC l\u201d that overlap slightly. Both spheres have a small dot in the center. Diagram b shows an H bonded to a C l with a single bond. A dipole and a positive sign are written above the H and a dipole and negative sign are written above the C l. An arrow points toward the C l with a plus sign on the end furthest from the arrow\u2019s head near the H.\" width=\"328\" height=\"225\" \/><\/a><figcaption class=\"wp-caption-text\"><strong>Figure 2.<\/strong> (a) The distribution of electron density in the HCl molecule is uneven. The electron density is greater around the chlorine nucleus. The small, black dots indicate the location of the hydrogen and chlorine nuclei in the molecule. (b) Symbols \u03b4+ and \u03b4\u2013 indicate the polarity of the H\u2013Cl bond.<\/figcaption><\/figure>\n<\/figure>\n<\/section>\n<section id=\"fs-idp14008096\">\n<h2>Electronegativity<\/h2>\n<p id=\"fs-idm42504736\">Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called <strong>electronegativity<\/strong>. Electronegativity is a measure of the tendency of an atom to attract electrons (or electron density) towards itself. It determines how the shared electrons are distributed between the two atoms in a bond. The more strongly an atom attracts the electrons in its bonds, the larger its electronegativity. Electrons in a polar covalent bond are shifted toward the more electronegative atom; thus, the more electronegative atom is the one with the partial negative charge. The greater the difference in electronegativity, the more polarized the electron distribution and the larger the partial charges of the atoms.<\/p>\n<p id=\"fs-idm49461456\"><a href=\"#CNX_Chem_07_02_ENTable\" class=\"autogenerated-content\">Figure 3<\/a> shows the electronegativity values of the elements as proposed by one of the most famous chemists of the twentieth century: Linus Pauling (<a href=\"#CNX_Chem_07_02_Pauling\" class=\"autogenerated-content\">Figure 4<\/a>). In general, electronegativity increases from left to right across a period in the periodic table and decreases down a group. Thus, the nonmetals, which lie in the upper right, tend to have the highest electronegativities, with fluorine the most electronegative element of all (EN = 4.0). Metals tend to be less electronegative elements, and the group 1 metals have the lowest electronegativities. Note that noble gases are excluded from this figure because these atoms usually do not share electrons with others atoms since they have a full valence shell. (While noble gas compounds such as XeO<sub>2<\/sub> do exist, they can only be formed under extreme conditions, and thus they do not fit neatly into the general model of electronegativity.)<\/p>\n<figure id=\"CNX_Chem_07_02_ENTable\"><figcaption>\n<figure style=\"width: 1300px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/chemistry\/wp-content\/uploads\/sites\/150\/2016\/05\/CNX_Chem_07_02_ENTable.jpg\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-content\/uploads\/sites\/387\/2018\/04\/CNX_Chem_07_02_ENTable-2.jpg\" alt=\"Part of the periodic table is shown. A downward-facing arrow is drawn to the left of the table and labeled, \u201cDecreasing electronegativity,\u201d while a right-facing arrow is drawn above the table and labeled \u201cIncreasing electronegativity.\u201d The electronegativity for almost all the elements is given.\" width=\"1300\" height=\"557\" \/><\/a><figcaption class=\"wp-caption-text\"><strong>Figure 3.<\/strong> The electronegativity values derived by Pauling follow predictable periodic trends with the higher electronegativities toward the upper right of the periodic table.<\/figcaption><\/figure>\n<\/figcaption><\/figure>\n<section id=\"fs-idm115216\">\n<h2>Electronegativity versus Electron Affinity<\/h2>\n<p id=\"fs-idp19127744\">We must be careful not to confuse electronegativity and electron affinity. The electron affinity of an element is a measurable physical quantity, namely, the energy released or absorbed when an isolated gas-phase atom acquires an electron, measured in kJ\/mol. Electronegativity, on the other hand, describes how tightly an atom attracts electrons in a bond. It is a dimensionless quantity that is calculated, not measured. Pauling derived the first electronegativity values by comparing the amounts of energy required to break different types of bonds. He chose an arbitrary relative scale ranging from 0 to 4.<\/p>\n<div id=\"fs-idm39789712\" class=\"textbox shaded\">\n<h3 class=\"title\">Linus Pauling<\/h3>\n<p id=\"fs-idp175623792\">Linus <strong class=\"no-emphasis\">Pauling<\/strong>, shown in <a href=\"#CNX_Chem_07_02_Pauling\" class=\"autogenerated-content\">Figure 4<\/a>, is the only person to have received two unshared (individual) Nobel Prizes: one for chemistry in 1954 for his work on the nature of chemical bonds and one for peace in 1962 for his opposition to weapons of mass destruction. He developed many of the theories and concepts that are foundational to our current understanding of chemistry, including electronegativity and resonance structures.<\/p>\n<figure id=\"CNX_Chem_07_02_Pauling\">\n<figure style=\"width: 325px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/chemistry\/wp-content\/uploads\/sites\/150\/2016\/05\/CNX_Chem_07_02_Pauling.jpg\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-content\/uploads\/sites\/387\/2018\/04\/CNX_Chem_07_02_Pauling-2.jpg\" alt=\"A photograph of Linus Pauling is shown.\" width=\"325\" height=\"394\" \/><\/a><figcaption class=\"wp-caption-text\"><strong>Figure 4.<\/strong> Linus Pauling (1901\u20131994) made many important contributions to the field of chemistry. He was also a prominent activist, publicizing issues related to health and nuclear weapons.<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-idp80917280\">Pauling also contributed to many other fields besides chemistry. His research on sickle cell anemia revealed the cause of the disease\u2014the presence of a genetically inherited abnormal protein in the blood\u2014and paved the way for the field of molecular genetics. His work was also pivotal in curbing the testing of nuclear weapons; he proved that radioactive fallout from nuclear testing posed a public health risk.<\/p>\n<\/div>\n<\/section>\n<section id=\"fs-idp9576080\">\n<h2>Electronegativity and Bond Type<\/h2>\n<p id=\"fs-idp186399536\">The absolute value of the difference in electronegativity (\u0394EN) of two bonded atoms provides a rough measure of the polarity to be expected in the bond and, thus, the bond type. When the difference is very small or zero, the bond is covalent and nonpolar. When it is large, the bond is polar covalent or ionic. The absolute values of the electronegativity differences between the atoms in the bonds H\u2013H, H\u2013Cl, and Na\u2013Cl are 0 (nonpolar), 0.9 (polar covalent), and 2.1 (ionic), respectively. The degree to which electrons are shared between atoms varies from completely equal (pure covalent bonding) to not at all (ionic bonding). <a href=\"#CNX_Chem_07_02_DeltaEN\" class=\"autogenerated-content\">Figure 5<\/a> shows the relationship between electronegativity difference and bond type.<\/p>\n<figure id=\"CNX_Chem_07_02_DeltaEN\">\n<figure style=\"width: 1300px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/opentextbc.ca\/chemistry\/wp-content\/uploads\/sites\/150\/2016\/05\/CNX_Chem_07_02_DeltaEN.jpg\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-content\/uploads\/sites\/387\/2018\/04\/CNX_Chem_07_02_DeltaEN-2.jpg\" alt=\"Two flow charts and table are shown. The first flow chart is labeled, \u201cElectronegativity difference between bonding atoms.\u201d Below this label are three rounded text bubbles, connected by a downward-facing arrow, labeled, \u201cZero,\u201d \u201cIntermediate,\u201d and \u201cLarge,\u201d respectively. The second flow chart is labeled, \u201cBond type.\u201d Below this label are three rounded text bubbles, connected by a downward-facing arrow, labeled, \u201cPure covalent,\u201d \u201cPolar covalent,\u201d and \u201cIonic,\u201d respectively. A double ended arrow is written vertically to the right of the flow charts and labeled, \u201cCovalent character decreases; ionic character increases.\u201d The table is made up of two columns and four rows. The header line is labeled \u201cBond type\u201d and \u201cElectronegativity difference.\u201d The left column contains the phrases \u201cPure covalent,\u201d \u201cPolar covalent,\u201d and \u201cIonic,\u201d while the right column contains the values \u201cless than 0.4,\u201d \u201cbetween 0.4 and 1.8,\u201d and \u201cgreater than 1.8.\u201d\" width=\"1300\" height=\"468\" \/><\/a><figcaption class=\"wp-caption-text\"><strong>Figure 5.<\/strong> As the electronegativity difference increases between two atoms, the bond becomes more ionic.<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-idm32214656\">A rough approximation of the electronegativity differences associated with covalent, polar covalent, and ionic bonds is shown in <a href=\"#CNX_Chem_07_02_DeltaEN\" class=\"autogenerated-content\">Figure 5<\/a>. This table is just a general guide, however, with many exceptions. For example, the H and F atoms in HF have an electronegativity difference of 1.9, and the N and H atoms in NH<sub>3<\/sub> a difference of 0.9, yet both of these compounds form bonds that are considered polar covalent. Likewise, the Na and Cl atoms in NaCl have an electronegativity difference of 2.1, and the Mn and I atoms in MnI<sub>2<\/sub> have a difference of 1.0, yet both of these substances form ionic compounds.<\/p>\n<p id=\"fs-idm28373728\">The best guide to the covalent or ionic character of a bond is to consider the types of atoms involved and their relative positions in the periodic table. Bonds between two nonmetals are generally covalent; bonding between a metal and a nonmetal is often ionic.<\/p>\n<p id=\"fs-idm38253568\">Some compounds contain both covalent and ionic bonds. The atoms in polyatomic ions, such as OH<sup>\u2013<\/sup>, NO<sub>3<\/sub><sup>\u2212<\/sup>, and NH<sub>4<\/sub><sup>+<\/sup>, are held together by polar covalent bonds. However, these polyatomic ions form ionic compounds by combining with ions of opposite charge. For example, potassium nitrate, KNO<sub>3<\/sub>, contains the K<sup>+<\/sup> cation and the polyatomic NO<sub>3<\/sub><sup>\u2212<\/sup> anion. Thus, bonding in potassium nitrate is ionic, resulting from the electrostatic attraction between the ions K<sup>+<\/sup> and NO<sub>3<\/sub><sup>\u2212<\/sup>, as well as covalent between the nitrogen and oxygen atoms in NO<sub>3<\/sub><sup>\u2212<\/sup>.<\/p>\n<div class=\"textbox shaded\" id=\"fs-idm8119280\">\n<h3>Example 1<\/h3>\n<p id=\"fs-idp117774608\">Bond polarities play an important role in determining the structure of proteins. Using the electronegativity values in <a href=\"#CNX_Chem_07_02_ENTable\" class=\"autogenerated-content\">Figure 3<\/a>, arrange the following covalent bonds\u2014all commonly found in amino acids\u2014in order of increasing polarity. Then designate the positive and negative atoms using the symbols \u03b4+ and \u03b4\u2013:<\/p>\n<p id=\"fs-idm8126928\">C\u2013H, C\u2013N, C\u2013O, N\u2013H, O\u2013H, S\u2013H<\/p>\n<p>&nbsp;<\/p>\n<p id=\"fs-idm21713856\"><strong>Solution<\/strong><br \/>\nThe polarity of these bonds increases as the absolute value of the electronegativity difference increases. The atom with the \u03b4\u2013 designation is the more electronegative of the two. <a href=\"#fs-idm2614240\" class=\"autogenerated-content\">Table 1<\/a> shows these bonds in order of increasing polarity.<\/p>\n<table id=\"fs-idm2614240\" class=\"span-all\" summary=\"This table has three columns and seven rows. The first row is a header row and it labels each column. The first column header is, \u201cBond,\u201d the second is, \u201ccapital delta E N,\u201d and the third is, \u201cPolarity.\u201d Under the \u201cBond\u201d column are the following: C bonds to H with a single bond; S bonds to H with a single bond; C bonds to N with a single bond; N bonds to H with a single bond; C bonds to O with a single bond; and O bonds to H with a single bond. Under the \u201ccapital delta E N\u201d columna are the values: 0.4; 0.4; 0.5; 0.9; 1.0; and 1.4. Under the \u201cPolarity\u201d column are the follwoing: C bonds to H with a single bond, there is a lowercase delta negative sign above the C and a lowercase delta positive sign over H; S bonds to H with a single bond, there is a lowercase delta negative sign over the S and a lowercase delta positive sign over the H; C bonds to N with a single bond, there is a lowercase delta positive sign over the C and a lowercase delta negative sign over the N; N bonds to H with a single bond, there is a lowercase delta negative sign over the N and a lowercase delta positive sign over the H; C bonds to O with a single bond, there is a lowercase delta positive sign over C and a lowercase delta negative sign over the O; and O bonds to H with a single bond, there is a lowercase delta negative sign over the O and a lowercase delta positive sign over the H.\">\n<thead>\n<tr valign=\"top\">\n<th>Bond<\/th>\n<th>\u0394EN<\/th>\n<th>Polarity<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr valign=\"top\">\n<td>C\u2013H<\/td>\n<td>0.4<\/td>\n<td>[latex]\\overset{\\delta -}{\\text{C}} - \\overset{\\delta +}{\\text{H}}[\/latex]<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>S\u2013H<\/td>\n<td>0.4<\/td>\n<td>[latex]\\overset{\\delta -}{\\text{S}} - \\overset{\\delta +}{\\text{H}}[\/latex]<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>C\u2013N<\/td>\n<td>0.5<\/td>\n<td>[latex]\\overset{\\delta +}{\\text{C}} - \\overset{\\delta -}{\\text{N}}[\/latex]<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>N\u2013H<\/td>\n<td>0.9<\/td>\n<td>[latex]\\overset{\\delta -}{\\text{N}} - \\overset{\\delta +}{\\text{H}}[\/latex]<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>C\u2013O<\/td>\n<td>1.0<\/td>\n<td>[latex]\\overset{\\delta +}{\\text{C}} - \\overset{\\delta -}{\\text{O}}[\/latex]<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>O\u2013H<\/td>\n<td>1.4<\/td>\n<td>[latex]\\overset{\\delta -}{\\text{O}} - \\overset{\\delta +}{\\text{H}}[\/latex]<\/td>\n<\/tr>\n<tr>\n<td colspan=\"3\"><strong>Table 1.<\/strong> Bond Polarity and Electronegativity Difference<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>&nbsp;<\/p>\n<p id=\"fs-idp64846816\"><em><strong>Test Yourself<\/strong><\/em><br \/>\nSilicones are polymeric compounds containing, among others, the following types of covalent bonds: Si\u2013O, Si\u2013C, C\u2013H, and C\u2013C. Using the electronegativity values in <a href=\"#CNX_Chem_07_02_ENTable\" class=\"autogenerated-content\">Figure 3<\/a>, arrange the bonds in order of increasing polarity and designate the positive and negative atoms using the symbols \u03b4+ and \u03b4\u2013.<\/p>\n<p>&nbsp;<\/p>\n<p><em><strong>Answer<\/strong><\/em><\/p>\n<table id=\"fs-idm25109184\" class=\"medium unnumbered\" summary=\"This table has three columns and five rows. The first row is a header row and it labels each column. The first column header is, \u201cBond,\u201d the second column header is, \u201cElectronegativity Difference,\u201d and the third column header is, \u201cPolarity.\u201d Under the column \u201cBond\u201d are the following: C bonds to C with a single bond; C bonds to H with a single bond; S i bonds to C with a single bond; and S i bonds to O with a single bond. Under the column \u201cElectronegativity Difference\u201d are the values: 0.0; 0.4; 0.7; and 1.7. Under the column \u201cPolarity\u201d are the following: nonpolar, C bonds to H with a single bond, there is a lowercase delta negative sign over C and a lowercase delta positive sign over H; S i bonds to C with a single bond, there is a lowercase delta positive sign over S i and a lowercase delta negative sign over C; and S i bonds to O with a single bond, there is a lowercase delta positive sign over S i and a lowercase delta negative sign over O.\">\n<thead>\n<tr valign=\"top\">\n<th>Bond<\/th>\n<th>Electronegativity Difference<\/th>\n<th>Polarity<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr valign=\"top\">\n<td>C\u2013C<\/td>\n<td>0.0<\/td>\n<td>nonpolar<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>C\u2013H<\/td>\n<td>0.4<\/td>\n<td>[latex]\\overset{\\delta -}{\\text{C}} - \\overset{\\delta +}{\\text{H}}[\/latex]<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Si\u2013C<\/td>\n<td>0.7<\/td>\n<td>[latex]\\overset{\\delta +}{\\text{Si}} - \\overset{\\delta -}{\\text{C}}[\/latex]<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Si\u2013O<\/td>\n<td>1.7<\/td>\n<td>[latex]\\overset{\\delta +}{\\text{Si}} - \\overset{\\delta -}{\\text{O}}[\/latex]<\/td>\n<\/tr>\n<tr>\n<td colspan=\"3\"><strong>Table 2.<\/strong><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/section>\n<\/section>\n<section id=\"fs-idm4705056\" class=\"summary\">\n<h2>Key Concepts and Summary<\/h2>\n<p id=\"fs-idp86079584\">Covalent bonds form when electrons are shared between atoms and are attracted by the nuclei of both atoms. In pure covalent bonds, the electrons are shared equally. In polar covalent bonds, the electrons are shared unequally, as one atom exerts a stronger force of attraction on the electrons than the other. The ability of an atom to attract a pair of electrons in a chemical bond is called its electronegativity. The difference in electronegativity between two atoms determines how polar a bond will be. In a diatomic molecule with two identical atoms, there is no difference in electronegativity, so the bond is nonpolar or pure covalent. When the electronegativity difference is very large, as is the case between metals and nonmetals, the bonding is characterized as ionic.<\/p>\n<\/section>\n<section id=\"fs-idp134316112\" class=\"exercises\">\n<div class=\"bcc-box bcc-info\">\n<h3>Exercises<\/h3>\n<p>1. Why is it incorrect to speak of a molecule of solid NaCl?<\/p>\n<p>2. Predict which of the following compounds are ionic and which are covalent, based on the location of their constituent atoms in the periodic table:<\/p>\n<p id=\"fs-idp137731248\">a) Cl<sub>2<\/sub>CO \u00a0 \u00a0 \u00a0 \u00a0\u00a0b) MnO \u00a0 \u00a0 \u00a0 \u00a0\u00a0c) NCl<sub>3 \u00a0 \u00a0 \u00a0 \u00a0\u00a0<\/sub>d) CoBr<sub>2 \u00a0 \u00a0 \u00a0 \u00a0\u00a0<\/sub>e) K<sub>2<\/sub>S \u00a0 \u00a0 \u00a0 \u00a0\u00a0f) CO<\/p>\n<p id=\"fs-idp70623968\">g) CaF<sub>2 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/sub>h) HI \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0i) CaO \u00a0 \u00a0 \u00a0 \u00a0\u00a0j) IBr \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0k) CO<sub>2<\/sub><\/p>\n<p>3. From its position in the periodic table, determine which atom in each pair is more electronegative:<\/p>\n<p id=\"fs-idm12153568\">a) Br or Cl \u00a0 \u00a0 \u00a0 \u00a0\u00a0b) N or O \u00a0 \u00a0 \u00a0 \u00a0\u00a0c) S or O \u00a0 \u00a0 \u00a0 \u00a0\u00a0d) P or S<\/p>\n<p id=\"fs-idp165919328\">e) Si or N \u00a0 \u00a0 \u00a0 \u00a0 \u00a0\u00a0f) Ba or P \u00a0 \u00a0 \u00a0 \u00a0\u00a0g) N or K<\/p>\n<p>4. From their positions in the periodic table, arrange the atoms in each of the following series in order of increasing electronegativity:<\/p>\n<p id=\"fs-idp63429168\">a) C, F, H, N, O \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0b) Br, Cl, F, H, I \u00a0 \u00a0 \u00a0 \u00a0 \u00a0c) F, H, O, P, S<\/p>\n<p id=\"fs-idp288482688\">d) Al, H, Na, O, P \u00a0 \u00a0 \u00a0 \u00a0\u00a0e) Ba, H, N, O, As<\/p>\n<p>5. Which atoms can bond to sulfur so as to produce a positive partial charge on the sulfur atom?<\/p>\n<p>6. Which is the most polar bond?<\/p>\n<p id=\"fs-idm10175808\">a) C\u2013C \u00a0 \u00a0 \u00a0 \u00a0\u00a0b) C\u2013H \u00a0 \u00a0 \u00a0 \u00a0\u00a0c) N\u2013H \u00a0 \u00a0 \u00a0 \u00a0\u00a0d) O\u2013H \u00a0 \u00a0 \u00a0 \u00a0\u00a0e) Se\u2013H<\/p>\n<p>7. Identify the more polar bond in each of the following pairs of bonds:<\/p>\n<p id=\"fs-idp20525952\">a) HF or HCl \u00a0 \u00a0 \u00a0 \u00a0\u00a0b) NO or CO \u00a0 \u00a0 \u00a0 \u00a0\u00a0c) SH or OH \u00a0 \u00a0 \u00a0 \u00a0\u00a0d) PCl or SCl<\/p>\n<p id=\"fs-idp183486032\">e) CH or NH \u00a0 \u00a0 \u00a0 \u00a0\u00a0f) SO or PO \u00a0 \u00a0 \u00a0 \u00a0 \u00a0\u00a0g) CN or NN<\/p>\n<p>&nbsp;<\/p>\n<p id=\"fs-idp58442352\"><strong>Answers<\/strong><\/p>\n<p id=\"fs-idm21247904\">1. NaCl consists of discrete ions arranged in a crystal lattice, not covalently bonded molecules.<\/p>\n<p id=\"fs-idm92647632\">2. ionic: b), d), e), g), and i); \u00a0 \u00a0 \u00a0covalent: a), c), f), h), j), and \u00a0k)<\/p>\n<p id=\"fs-idm10691120\">3. a) Cl \u00a0 \u00a0 \u00a0 b) O \u00a0 \u00a0 \u00a0 \u00a0c) O \u00a0 \u00a0 \u00a0 d) S \u00a0 \u00a0 \u00a0 e) N \u00a0 \u00a0 \u00a0 \u00a0 f) P \u00a0 \u00a0 \u00a0 g) N<\/p>\n<p id=\"fs-idm60378112\">4. a) H, C, N, O, F \u00a0 \u00a0 \u00a0 b) H, I, Br, Cl, F \u00a0 \u00a0 \u00a0 c) H, P, S, O, F \u00a0 \u00a0 \u00a0 d) Na, Al, H, P, O \u00a0 \u00a0 \u00a0 e) Ba, H, As, N, O<\/p>\n<p id=\"fs-idm121823728\">5. N, O, F, and Cl<\/p>\n<p id=\"fs-idm3310160\">6. \u00a0O-H<\/p>\n<p>7. a) HF \u00a0 \u00a0 \u00a0 b) CO \u00a0 \u00a0 \u00a0 c) OH \u00a0 \u00a0 \u00a0 d) PCl \u00a0 \u00a0 \u00a0 e) NH \u00a0 \u00a0 \u00a0 f) PO \u00a0 \u00a0 \u00a0 \u00a0g) CN<\/p>\n<\/div>\n<\/section>\n<div>\n<h2>Glossary<\/h2>\n<p><strong>bond length:\u00a0<\/strong>distance between the nuclei of two bonded atoms at which the lowest potential energy is achieved<\/p>\n<p><strong>covalent bond:\u00a0<\/strong>bond formed when electrons are shared between atoms<\/p>\n<p><strong>electronegativity:\u00a0<\/strong>tendency of an atom to attract electrons in a bond to itself<\/p>\n<p><strong>polar covalent bond:\u00a0<\/strong>covalent bond between atoms of different electronegativities; a covalent bond with a positive end and a negative end<\/p>\n<p><strong>pure covalent bond:\u00a0<\/strong>(also, nonpolar covalent bond) covalent bond between atoms of identical electronegativities<\/p>\n<\/div>\n","protected":false},"author":330,"menu_order":3,"template":"","meta":{"pb_show_title":"on","pb_short_title":"9.2 Covalent Bonding","pb_subtitle":"","pb_authors":[],"pb_section_license":"cc-by"},"chapter-type":[],"contributor":[],"license":[50],"class_list":["post-1549","chapter","type-chapter","status-publish","hentry","license-cc-by"],"part":1538,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-json\/pressbooks\/v2\/chapters\/1549","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-json\/wp\/v2\/users\/330"}],"version-history":[{"count":7,"href":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-json\/pressbooks\/v2\/chapters\/1549\/revisions"}],"predecessor-version":[{"id":4704,"href":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-json\/pressbooks\/v2\/chapters\/1549\/revisions\/4704"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-json\/pressbooks\/v2\/parts\/1538"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-json\/pressbooks\/v2\/chapters\/1549\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-json\/wp\/v2\/media?parent=1549"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-json\/pressbooks\/v2\/chapter-type?post=1549"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-json\/wp\/v2\/contributor?post=1549"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/chem1114langaracollege\/wp-json\/wp\/v2\/license?post=1549"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}