3.12: Chemical Bonds

The atoms in molecules are held together by chemical bonds, such as covalent and ionic bonds. To understand these bonds, we need to return to the basic structure of the atom.

An atom’s electrons orbit the nucleus in pathways called electron shells (shown as circles in Figure 3.1). Each shell can hold a limited number of electrons. Electrons fill the shells in an orderly fashion, starting with the shell nearest the nucleus.

The innermost shell holds a maximum of 2 electrons. If there are more than 2, the electrons form another shell in an outward direction; this second shell holds a maximum of 8. After the second shell is filled, a third shell is formed, and so on. Recall that the number of electrons in an atom is different for each element—a unique number of electrons (negative charges) to balance the unique number of protons (positive charges) that characterizes each element (Figure 3.1).

Atoms like helium and neon (Figure 3.1) are stable—they don’t combine with other atoms because their electron shells are completely full (only the outer shell can be unfilled; the inner shells always are full).* Helium (used to fill balloons) and neon (in neon signs) are called noble gases—noble because they don’t accept, donate, or share electrons with other atoms. In other words, they don’t combine (form bonds) with other atoms, not even with another neon or helium atom; their outer electron shells are already full.

Most atoms don’t have a full outer electron shell, and fill it by accepting, donating, or sharing electrons with other atoms. In other words, these atoms readily combine/bond with other atoms to form molecules.

^{*Valence (from Latin valentice: strength/capacity) is the number of electrons an atom needs to lose, share, or gain for a full outer electron shell. The + or - is the resulting change in charge, e.g., helium 0, sodium +1 (loses its 1 electron in its outer shell, as does hydrogen +1), chlorine -2 (has 6 in outer shell, so gains 2, as does oxygen -2). (see Atoms in Figure 3.1).}