halogen react with alkali metals because of their valence electrons. Halogens are just one electron short of being a stable wanna be "noble gas" so they "take or borrow" an electron. alkali metals have one too many valence electrons to be a noble gas, so they "give or lend" an electron.
this is why they are so compatible.
EXAMPLE
NaCl= Na+ +Cl-
Sodium(Na) "gives" its extra electron in order to become stable and Chlorine(Cl) "takes" Na extra electron in order to become stable
