Look at electronegativity and the atom's likeliness to become isoelectronic with a noble gas.
Some elements have this "desire", as one may put it, to react in order to have a full octet ( 8 valence electrons). The noble gases do not like to react at all since they already have a full octet. The halogens and alkali metals though react aggressively.
This is due to the halogens having high electronegativities from having more protons than those in their row , and the halogens really want to become isoelectronic ( same number of electrons) with the noble gases.
If the alkali metals donate that one electron to the halogen they too become isoelectronic with the noble gases.
( Atoms want to be isoelectronic with noble gases because noble gases are stable).
As moving inward this reactivity decreases, as the strength to acquire more electrons decreases. While electronegativity is important to distinguish reactivity the alkali metals have a small number of electrons and that is easier to lose . So simply look at the likeliness to lose or gain electrons to become isoelectronic with a noble gas.