Exceptions to the Octet Rule

The octet rule is almost always obeyed by the elements in the upper right corner of the periodic table: C, N, O, and F. Although molecules are known that do not satisfy the octet rule, e.g., methyl radical, CH₃, they are unstable, highly reactive, and usually have very short lifetimes. Molecules with atoms from the left side of the periodic table usually show exceptions to the octet rule. Thus in BF₃, the B has 3 valence electrons, just enough to form 3 electron pair bonds with F. We could formally satisfy the octet rule by invoking 3 resonance structures, each with 1 B=F and 2 B-F bonds, but this would result in a formal charge of -1 on B and, on the average, +1/3 on F. Since F is much more electronegative than B, this arrangement seems unlikely and we accept the fact that B does not obey the octet rule.

Atoms from the second row of the periodic table—Si, P, S, and Cl—often form molecules in which one of these atoms has a share of more than 8 electrons, e.g., PCl₅ and SF₆. This kind of exception to the octet rule is usually rationalized by saying that these atoms can use their empty 3d orbitals to assist in bond formation, thus allowing for more than 8 electrons.