Which determines the reactivity of an alkali metal? its boiling and melting points
the shininess of its surface
the number of protons it has
its ability to lose electrons

The reactivity of an alkali metal is determined by its ability to loose electrons.

The alkali metals are highly electropositive. They easily loose electron to form a univalent positive ion.

This ability to form a univalent positive ion increases down the group hence cesium is the most electropositive element in nature.

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