Breaking the Octet Rule

Some elements when they form bonds can break the octet rule. This is a rule that says that atoms must have a full outer shell of eight electrons. Sadly this is not always the case.

The previous example, boron trifluoride is an excellent example.

The standard example given however is beryllium chloride (BeCl₂).

Notice that the Be only has 4 electrons (two of them are shared).

There are other examples of this in period 2, but no period 2 element can actually exceed 8 electrons in its outer shell.

Why can some elements exceed this previously hard and fast rule?

The larger orbitals and increased numbers of sub-levels and energy levels means the energy difference between sub-levels and orbitals becomes less as the period number increases. In other words it becomes easier to move electrons from one sub-level to another.

So oxygen in period 2 is limited to 8 electrons in its outer shell, but sulphur can have up to 12.

This also applies to phosphorous which can form PCl₃ or PCl₅.