Answer:
alkali metals: 1 valence electron, very reactive metals
alkaline earth metals: 2 valence electrons, reactive metals
halogens: 7 valence electrons, very reactive nonmetals
noble gases: 8 valence electrons, very little reactivity
Explanation:
The groups increase by 1 valence electron as you move from left to right across the periodic table, skipping over the transition metals. The closer an element is to having a full octet of 8 valence electrons, the more reactive it will be. The alkali metals and halogens are both just 1 away from an octet -- alkali lose an electron, halogens gain one. This makes them very reactive.
