So far we’ve seen that elements typically follow the octet rule, meaning that they form compounds in which they have access to eight valence electrons. Keep in mind that atoms form bonds to reach a lower energy,and that just so happens to be when there are 8 electrons in the valence shell. However, there are, three exceptions to the octet rule: 1) Compounds that have odd numbers of electrons do not follow the octet rule. 2) There are also compounds in which certain atoms lack a complete octet. 3) Finally, there are compounds in which atoms contain more than an octet of electrons. We’ll walk through each case and examine compounds that disobey the octet rule in each of these ways.
Molecules with odd numbers of electrons violate the octet rule. Molecules that have odd numbers of electrons are relatively rare, but they do exist, and they even relevant to living systems. One example is nitric oxide (NO) and contains a total of 11 valence electrons (five electrons from nitrogen and six electrons from oxygen). If you try to draw out a Lewis structure for this molecule, you’ll find that it’s impossible for both atoms to have complete octets (see video). Despite its unusual Lewis structure, NO is actually an important vertebrate signaling molecule. It is created by cells and used as a signal that triggers the relaxation of the muscle cells that line blood vessels. In fact, nitroglycerine, a medication given to treat chest pain, works by increasing the levels of nitric oxide in the body, thus causing blood vessels to dilate (expand). Generally speaking, these odd-electron molecules tend to be fairly reaction (which is why they’re somewhat rare to find).
Some molecules featuring boron and beryllium have structures in which boron
and beryllium have incomplete octets. This is a pretty rare situation since
compounds with boron and beryllium aren’t encountered all that frequently. An
example it boron trifluoride (BF
The reactivity patterns of boron trifluoride are consistent with this bonding arrangement. Boron reacts readily with molecules like water and ammonia that can donate a lone pair, providing a fourth bond to boron and completing its octet.
The most common exceptions to the octet rule involve compounds in which an atom has more than eight electrons (it exceeds an octet). These atoms are also called hypervalent, and this is sometimes referred to as octet as expansion. Octet expansion only occurs for atoms in periods 3 and beyond on the periodic table (period 2 elements cannot undergo octet expansion). When octet expansion occurs, the central atom is often bonded to numerous small, electronegative atoms (like O, Cl, or F).