What is the electronic configuration of Chlorine and of a chloride ion and explain why Chlorine is 'happiest' as a chloride ion.

Cl - 1s22s22p63s23p5

Cl- - 1s22s22p63s23p6

The 3p orbital is slightly unstable because of the single unpaired electron which is at a higher energy and wants to pair up, the 3p orbital is more stable when the shell is either half full or completely full, so the chloride ion is more stable due to it having a full 3p orbital.