3.3: "Exceptions" to the Octet Rule

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The maximum number of electrons possible in the valence shell of the second row elements is eight. However, the elements of the third row, such as phosphorus and sulfur, can form stable systems by sharing eight or more electrons. The presence of d-orbitals, which can accommodate up to ten electrons, makes this possible.

Now, back to the second row, what happens when the first nonmetal, boron (Z=3), combines with hydrogen? By repeating the process outlined before for carbon, nitrogen, oxygen, and fluorine, we conclude that boron needs to bond to 5 hydrogen atoms to fulfill the octet rule. The problem is that with only three electrons in the valence shell this is impossible:

The only possibility for boron is to bond to three hydrogen atoms, in which case it forms a compound (borane, BH₃) that does not fulfill the octet rule. The compound actually exists, but it is highly reactive, that is to say, unstable. Substances such as BH₃ are referred to as electron-deficient molecules, and are very reactive towards electron-rich substances.

Aluminum, which is also in group III, exhibits similar behaviour.