Based on the electron configurations and orbital spin diagrams for elements sodium and chlorine

Question

explain why sodium metal reacts with chlorine gas to give the compound sodium chloride 

Answer 1

This may best be answered by looking at the positions of Na and Cl in the periodic table.  Na is in group 1, meaning it has a single electron in its valence band (n = 3).  Cl is in group 17, meaning its valence band is one electron short of being filled.  When Na and Cl atoms come together, it is natural for Na to give up its single outer electron, leaving behind the ion Na⁺ with a filled outer shell (n = 2, since the electron in n = 3 is gone), and for Cl to acquire that electron from Na, resulting in the ion Cl^-, with its outer shell filled. The 2 ions have opposite charges and attract to form NaCl, which is neutral overall.