1.2. The Octet Rule
The atoms that participate in covalent bonding share electrons in a way that enables them to acquire a stable electronic configuration, or full valence shell. Consider the ground-state electron configuration of carbon (Figure 1.5 above). Carbon has 4 valence electrons already and needs another 4 electrons to fill its valence orbitals, for a total of 8 valence electrons. This holds for the other elements in that row, such as nitrogen and oxygen, as well as the halogens (F, Cl, Br, and I). We call this the Octet Rule.
Octet Rule: Atoms in the first row (C, N, O) and the halogens (F, Cl, Br, I) look to fill their valence shells with EIGHT electrons – NEVER more, VERY RARELY less.
It is very important to remember that one commonly encountered element does not follow this pattern: hydrogen. Hydrogen only has one valence orbital (the 1s orbital) and as a result looks to fill its valence shell with TWO electrons, not eight.
Each covalent bond an atom makes with another atom functionally allows it to gain an electron in its valence orbitals (this works for the other atom as well). As a result, the number of bonds that an atom normally makes is directly equal to the number of additional valence electrons it needs to fill its valence shell. More specifically…
Carbon has 4 valence electrons.
It wants 4 more electrons to have 8 valence electrons.
Therefore Carbon (usually) makes 4 bonds.
Nitrogen has 5 valence electrons.
It wants 3 more electrons to have 8 valence electrons.
Therefore Nitrogen (usually) makes 3 bonds.
Oxygen has 6 valence electrons.
It wants 2 more electrons to have 8 valence electrons.
Therefore Oxygen (usually) makes 2 bonds.
Fluorine/Chlorine/Bromine/Iodine has 7 valence electrons.
It wants 1 more electron to have 8 valence electrons.
Therefore Fluorine/Chlorine/Bromine/Iodine (usually) makes 1 bond.
Hydrogen has 1 valence electron.
It wants 1 more electron to have 2 valence electrons.
Therefore Hydrogen (usually) makes 1 bond.
This pattern can occasionally (but not always) be extended to elements in the next row as well. For example, silicon (Si) is in Group 4 like carbon and usually makes 4 bonds. Phosphorous (P) is in Group 5 like nitrogen and often makes 3 bonds, though sometimes it makes more (e.g. phosphoric acid, H3PO4). Sulfur (S) is in Group 6 like oxygen and often makes 2 bonds, though sometimes it makes more (e.g. sulfuric acid, H2SO4). Why elements outside of the first row do not always follow the Octet Rule is beyond the scope of this text, but involves the smaller energy differences between available orbitals once they are sufficiently high in energy.