What makes an element active?

Answer 1

I assume you mean chemical reactivity. The most active elements are:
(1) Above level 2 in the periodic table (starting with sodium
(2) Are either in the alkali metals or the halogen non-metals.
Alkali metals have 1 electron in an unsaturated s sub-shell and can readily shed it (to a cationic element) to empty the sub-shell. The halogens lack 1 electron to fill their outer-most p sub-shell, and readily accept electrons from anions.

Answer 2

unstable electrons! ;)