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Why are alkali metals highly reactive?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — alkali metal Modern version of the periodic table of the elements (printable).

Alkali metals are highly reactive primarily due to their electronic configuration. Each alkali metal atom possesses a single electron in its outermost shell, known as a valence electron.12

This valence electron is held much more weakly than those in inner shells, resulting in low ionization energy—the energy required to remove an electron from an atom. Because this electron is easily lost, alkali metals readily form singly charged positive ions (cations) when reacting with other substances.1

Furthermore, as a group, alkali metals have the lowest electronegativity of all elements, meaning they have a very weak tendency to attract electrons to themselves. These combined factors—the presence of a single, loosely bound valence electron and low electronegativity—make alkali metals exceptionally reactive, causing them to readily participate in chemical reactions with nonmetals, water, and atmospheric oxygen.1342