Breaking the octet rule: SO₂, PCl₅ and SF₆

By the end of this lesson

By the end you can explain why elements in Period 3 can expand their octet, and count the electrons around the central atom in $\text{SO}_2$ , $\text{PCl}_5$ and $\text{SF}_6$ .

A family mystery

Nitrogen and phosphorus are both in Group 15. Both make a trichloride: $\text{NCl}_3$ and $\text{PCl}_3$ .

But only phosphorus makes a pentachloride. $\text{PCl}_5$ is a real chemical you can buy; $\text{NCl}_5$ doesn't exist. Same group, different rules. Why?

A simple shell model

In the simple shell-capacity model, Period 2 atoms (N, O, F) have 2s and 2p orbitals in their outer shell: at most 8 electrons.
The traditional A-level account includes the empty 3d orbitals in the Period 3 shell. It uses them to account for more than 8 electrons around P or S in the examples below.

Having more than 8 electrons around the central atom is called expanding the octet. Treat the 3d explanation as a simplified shell model, not a literal modern description of the bonds.

Quick check

Multiple choiceQuestion 1 · Apply

Answer the question above to continue.