Why do halogens react so easily with Alkali Metals?

The electron configuration is why they react.

The Alkali metals have one electron in their outer shell and to complete their outer shell need 7 more electrons. They can give, take or share electrons with other atoms.

This is when the Halogens come in handy. The Halogens have 7 electrons in their outer shells, and need one more to complete the outer shell.

These can react very easily because they have both the perfect amounts to fill their outer shells and become compounds. Other atoms with other amounts to become atoms not ions needs another element to react with.

Hope this helps, Matt.