* Group 1: Alkali Metals (Li, Na, K, Rb, Cs, Fr) - These metals have only one valence electron, making them highly reactive. They readily lose this electron to form +1 cations, reacting vigorously with water, oxygen, and other nonmetals.

* Group 2: Alkaline Earth Metals (Be, Mg, Ca, Sr, Ba, Ra) - These metals have two valence electrons and are also very reactive, although less so than alkali metals. They react readily with water and oxygen, forming +2 cations.

Other groups with notable reactivity:

* Group 17: Halogens (F, Cl, Br, I, At) - These nonmetals have seven valence electrons and are highly reactive. They readily gain one electron to form -1 anions, reacting vigorously with metals and other nonmetals.

* Group 16: Chalcogens (O, S, Se, Te, Po) - These nonmetals have six valence electrons and are less reactive than halogens but still highly reactive. They readily gain two electrons to form -2 anions.

It's important to note that reactivity is a relative concept, and even within a group, the reactivity changes. For instance, fluorine (F) is the most reactive halogen, while astatine (At) is the least reactive.