Question

How do you determine the electron configuration of Cl?

Answer 1

In most cases, you can tell the electron configuration from the position in the Periodic Table, but sometimes there are exceptions. Fortunately, Cl is easy.

Explanation:

Chlorine (Cl) is element 17, and it lies in the third row (period) in the next-to-last column. The previous rare gas element is neon (Ne, element 10), which has a closed-shell configuration:
`[Ne] = (1s)^2(2s)^2(2p)^6`

Chlorine has this core configuration, plus 7 valence electrons in the `3s` and `3p` shells. It has only 5 `3p` electrons and thus is one short of the closed shell configuration corresponding to argon (element 18).

`[Cl] = [Ne] (3s)^2 (3p)^5 = (1s)^2 (2s)^2 (2p)^6 (3s)^2 (3p)^5`