# How do you determine the electron configuration of Cl?

Jul 5, 2016

#### Answer:

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:
$\left[N e\right] = {\left(1 s\right)}^{2} {\left(2 s\right)}^{2} {\left(2 p\right)}^{6}$

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

$\left[C l\right] = \left[N e\right] {\left(3 s\right)}^{2} {\left(3 p\right)}^{5} = {\left(1 s\right)}^{2} {\left(2 s\right)}^{2} {\left(2 p\right)}^{6} {\left(3 s\right)}^{2} {\left(3 p\right)}^{5}$