after a quick scan at the q ...
Looking at the periodic table helps a lot here, I think.
elements 1 and 17 are very very reactive. 1 are one above the 18 elements which are inert. 17 are one below the 18 elements which are inert. So, if they can react in with something, they'll gain an inert electron distrbution.
Na is group 1. Cl is group 17, NaCl gives Na group 18 electron configuration, whilst Cl also get group 18 config.
