Ab initio calculations are used to examine the stability of various isomers of benzene and naphthalene after substitution of pairs of carbon atoms by the isoelectronic boron-nitrogen (BN) tandem. It is found that stability is enhanced by keeping the B and N atoms adjacent to one another. When multiple pairs of BN are present, these pairs prefer to be consecutive with one another. The energy is also lowered by maintaining the heteroatoms on the same ring of naphthalene. Another factor is the preference that consecutive carbon atoms be grouped into even numbers. Successively higher degrees of substitution lead to a regular drop in the molecular valency, as well as increased hardness.