Here's why:
* Alkali metals are found in Group 1 of the periodic table. They have one valence electron and readily lose it to form a +1 cation.
* Halogens are found in Group 17 of the periodic table. They have seven valence electrons and readily gain one electron to form a -1 anion.
Because they have drastically different chemical properties and electron configurations, no element can be both an alkali metal and a halogen.
