Chapter 3. Atoms, Molecules, and Ions

3.6 End of Chapter Problems

1. How many electrons does it take to make the mass of one proton?

2. Dalton’s initial version of the modern atomic theory says that all atoms of the same element are the same. Is this actually correct? Why or why not?

3. Give complete atomic symbols for the three known isotopes of hydrogen.

4. Use its place on the periodic table to determine if indium, In, atomic number 49, is a metal or a nonmetal.

5. Americium-241 is a crucial part of many smoke detectors. How many neutrons are present in its nucleus

6. Determine the atomic mass of ruthenium from the given abundance and mass data.

Ruthenium-96 5.54% 95.907 u
Ruthenium-98 1.87% 97.905 u
Ruthenium-99 12.76% 98.906 u
Ruthenium-100 12.60% 99.904 u
Ruthenium-101 17.06% 100.906 u
Ruthenium-102 31.55% 101.904 u
Ruthenium-104 18.62% 103.905 u

7. One atomic mass unit has a mass of 1.6605 × 10−24 g. What is the mass of one atom of sodium?

8. One atomic mass unit has a mass of 1.6605 × 10−24 g. What is the mass of one molecule of H2O?

9. From their positions on the periodic table, will Cu and I form a molecular compound or an ionic compound?

10. Mercury is an unusual element in that when it takes a 1+ charge as a cation, it always exists as the diatomic ion. a)  Propose a formula for the mercury(I) ion.  b)What is the formula of mercury(I) chloride?

11. The uranyl cation has the formula UO22+. Propose formulas and names for the ionic compounds between the uranyl cation and F, SO42−, and PO43−.

12. Using a periodic table, identify the element symbol and group name for the following elements and identify it as either a metal, non-metal or metalloid:
a) Rubidium      b)  Strontium      c)  Californium      d)  Aluminum (no group name required)
e)  Iodine           f)   Krypton        g)  Tin (no group name required)

13. List 5 transition metals with their name, element symbol and atomic number.

14. What is a diatomic element? Give several examples.

15. Explain the experiment and findings of each of the following scientists:
a) Rutherford       b) J. J. Thompson      c)  Millikan

16. Use Dalton’s theory to explain why potassium nitrate from India or Italy has the same mass percents (or ratios) of K, N and O.

17. Street drugs are often mixed with an inactive substance, such as ascorbic acid (vitamin C). By separating a drug mixture into component substances and calculating the mass of vitamin C per gram of sample, government chemists can track the drug’s distribution. For example, if different cocaine samples from New York, L. A. and Paris all contain 0.6384 g of vitamin C per gram of sample, they likely come from a common source. Is this street sample a compound, element or mixture? In this case, does the constant mass ratio of the components exemplify the law of constant composition? Explain.

18. For each of the following, based on the info given, fill in the blanks:

a) Zn, Z = 30, neutrons = 34, atomic number = ?, protons = ?
b) Protons = 53, neutrons = 74, symbol = ?, mass number = ?, A = ?, Z = ?
c)  Eu, Z = ?, A = 153, protons = ?, neutrons = ?

19.  Complete the following table:

Symbol Element Protons Neutrons Electrons Mass Number


Atomic number


Magnesium 13
18 37 17
Nickel 28 60

20.  Write the complete atomic symbol for each of the following isotopes and state the number of protons, electrons and neutrons for each:
a) Fluorine with a mass number of 18       b)  Atomic number of 7 and 8 neutrons
c)  Z = 18, neutrons = 22                            d)  He, with A = 3
e)  Z = 82, A = 207                                     f)  Beryllium with 5 neutrons

21.  How many electrons are present in the following ions:

a)Fe3+   b) Cu+   c) Ni2+   d) Br   e) Eu3+   f) P3-

22.  List the number of protons, electrons and neutrons for the following:

a)  [latex]_{82}^{208}\text{Pb}^{2+}[/latex]       b)  [latex]_{7}^{14}\text{N}^{3-}[/latex]

23.  List the number of atoms present in a molecule (or formula unit) of:
a) NaNO3 b) C2H5OH    c) Fe(ClO4)3

24. Gallium has 2 naturally occurring isotopes, 69Ga (isotopic mass = 68.9256 amu, percent abundance = 60.11%) and 71Ga (isotopic mass = 70.9247 amu, percent abundance = 39.89%). Calculate the average atomic mass of Gallium.

25. Chlorine has 2 naturally occurring isotopes, 35Cl (isotopic mass = 34.9689 amu) and 37Cl (isotopic mass = 36.9659). If the average atomic mass of Cl is 35.4527 amu, what is the percent abundance of each isotope?

26. The two naturally occurring isotopes of nitrogen have masses of 14.0031 amu and 15.0001 amu, respectively. Determine the percentage of 15N atoms in naturally occurring nitrogen with average atomic mass of 14.0067 amu.  (Hint, the TOTAL abundance of the two isotopes must = 100%.)



1.  About 1,800 electrons

2.  It is not strictly correct because of the existence of isotopes.

3.  [latex]_{1}^{1}\text{H}[/latex], [latex]_{1}^{2}\text{H}[/latex], and [latex]_{1}^{3}\text{H}[/latex]

4.  It is a metal.

5.  146 neutrons

6.  101.065 u

7.  3.817 × 10−23 g

8.  2.991 × 10−23 g

9.  ionic

10.  a) Hg22+        b) Hg2Cl2

11.  Uranyl fluoride, UO2F2; uranyl sulfate, UO2SO4; uranyl phosphate, (UO2)3(PO4)2

12.  a) Rb, alkali metal, metal      b) Sr, alkaline earth metal, metal      c) Cf, actinide, metal
d) Al (no group name required), metal      e) I, halogen, non-metal      f) Kr, nobel gas, non-metal
g) Sn (no group name required), metal

13.  (Variety of answers possible)

14.  An element that exists in the natural state as 2 atoms bonded together in a molecule. Examples include H2, O2, N2, halogens

15.  a) Rutherford: performed an experiment where he shot alpha particles at gold foil. He observed that many alphas were deflected and some bounced back, suggesting that Thompson’s view of the atom was incorrect. Instead, he proposed that atom had a nucleus of positive charge, surrounded by a “sea” of negative charge (electrons)

b) J. J. Thompson: used a cathode ray tube (consisting of a negatively charged beam), and calculations regarding the deflection of the beam in a magnetic or electric field, to calculate the mass/charge ratio of the electron.

c) Millikan: performed an experiment where, in a mist of oil in air, droplets were covered with electrons (resulting from an X-ray hitting gas molecules in the air). He measured the rate of fall of these charged oil droplets through an electric field, and from this he determined the actual charge of an electron, and thus (in conjunction with Thompson’s work) the mass of an electron.

16.  The law of constant composition tells us that a given compound will always have the same mass percents of its components. Dalton used this idea, and his concept of the atom, to form his forth postulate, which states that atoms combine in fixed ratios of whole numbers to form compounds. If ratios remain the same, and the masses of each constituent atom are the same, the mass percents will remain the same regardless of the size of the sample.

17.  It is a mixture. It does not exemplify the law of constant composition. The “constant” composition results from the fact that they are all part of the same mixture (same source), but the composition COULD have varied and still produced a mixture of cocaine and vitamin C (with a different amount of vitamin C per sample) depending on the manufacturer.

18.  a) Zn: atomic number = 30, protons = 30
b) symbol = I (or a more complete symbol = [latex]_{53}^{127}\text{I}[/latex]),  mass number = 127, A = 127, Z = 53
c) Eu: Z = 63, protons = 63, neutrons = 90


Symbol Element Protons Neutrons Electrons Mass Number


Atomic number


38Ar Argon 18 20 18 38 18
 [latex]_{12}^{25}\text{Mg}[/latex] Magnesium  12 13  12  25  12
 [latex]_{17}^{37}\text{Cl}^{-}[/latex]  Chlorine  17  20 18 37 17
[latex]_{27}^{60}\text{Co}[/latex]  Cobalt  27  33  27  60  27
[latex]_{28}^{60}\text{Ni}[/latex] Nickel 28  32 28 60  28

20.  a) 189F, p = 9, e = 9, n = 9             b) 157N, p = 7, e = 7, n = 8
c) 4018Ar, p = 18, n = 22, e = 18          d) 32He, p = 2, e = 2, n = 1
e)  20782Pb, p = 82, e = 82, n = 125      f) 94Be, p = 4, n = 5, e = 4

21.  a) 23      b) 28       c) 26       d) 36      e) 60       f) 18

22.  a) p = 82, n = 126, e = 80         b) p = 7, n = 7, e = 10

23.  a) 1 atom of Na, 1 atom of N and 3 atoms of O  (5 atoms total)
b) 2 atoms of C, 6 atoms of H and 1 atom of O  (9 atoms total)
c) 1 atom of Fe, 3 atoms of Cl and 12 atoms of O  (16 atoms total)

24.  69.72 amu

25.  75.774% 35Cl and 24.226% 37Cl

26.  0.36% 15N


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