Chapter 2: Chemical bonds and molecules

A molecule is matter that contains more than one atom. Molecules that contain more than one element in a fixed ratio are called compounds. Atoms in molecules and compounds are held together by chemical bonds. Chemical bonds are forces between atoms that hold them together.

The outermost electron shell of an atom is called the valence shell and the electrons within that shell are called valence electrons. Recall: the valence electrons are the highest energy electrons. The valence electrons may be transferred or shared with other atoms. The valence number of an element is the number of electrons required to fill the valence shell.

Reactive elements are elements with incomplete valence shells. For example, hydrogen (H) has an atomic number of 1. The valence shell is the first shell and that shell contains only 1 electron. The first shell can accommodate two electrons, so the valence shell is incomplete and H is reactive. Generally, these atoms will share or transfer valence electrons easily. In contrast, inert elements are elements with full or complete valence shells. These atoms do not share or transfer electrons easily. For example, helium (He) has an atomic number of two. The valence shell of He is the first shell and it contains the maximum two electrons. Therefore, He is inert.