1)  alkali metals only have 1 electron in outer level, halogens have 7 
2)  alkali metals form cations (+ ions), halogens form anions (- ions) 
3)  alkali metals are all solid at standard temp pressure. halogens are gasses.
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The differences between alkali metals and halogens are as follows:-
1. Alkali metals form cations while Halogens form anions.
2. Alkali metals are solids while Halogens are gases.
3. Alkali metals have 1 electron in their outermost orbit (shell) while Halogens have 7 electrons in their outermost orbit (shell).