How do you determine the electron configuration of Cl?

1 Answer
Jul 5, 2016

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.


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#