Here's why:

* Valence electrons: These are the electrons in the outermost shell of an atom, and they are the ones involved in chemical bonding.

* Alkali metals: These are the elements in Group 1 of the periodic table (Li, Na, K, Rb, Cs, Fr).

* Electron configuration: Alkali metals have one electron in their outermost s orbital. For example, sodium (Na) has the electron configuration 1s²2s²2p⁶3s¹, where the 3s¹ electron is the valence electron.

This single valence electron is why alkali metals are highly reactive. They readily lose this electron to form a +1 cation, which is why they are always found in ionic compounds.