What is the electron configuration of Cl-?

Based on the periodic table, the atomic number (#Z# of chlorine is 17. Since the atomic number is always equal to the number of protons or

#Z# = number of protons

and in ground state (no charge), the number of protons is equal to the number of electrons, then

#Z# = number of protons = number of electrons (ground state)

Therefore, the ground state electron configuration of the element, chlorine is:

#1s^2# #2s^2# #2p^6# #3s^2# #3p^5# = #color (green) "17 electrons"# (just add the superscripts)

For a ground state element to become an ion, it has to either gain or lose an electron in its outermost orbital. Elements usually do this in order to gain the electron stability of the noble gases (octet rule).

Now, for you to be not confuse, you always have to remember that:

(1) electron gain will result to a #color (red) "negative"# charge (#color (red) (-)#), and

(2) electron loss will result to a #color (blue) "positive"# charge (#color (blue) (+)#),

The chloride ion, #Cl^-#, has a charge of #color (red) (-1)#, meaning, it had gained 1 electron in its outermost orbital. The overall number of electrons is now 18.

Thus, the electron configuration for #Cl^-# should be

#1s^2# #2s^2# #2p^6# #3s^2# #3p^6# = #color (green) "18 electrons"#