Why do alkali metals have low ionization energy?

Alkali Metals: Alkali metals are the elements of group 1 because these elements react with water to form an alkaline solution with the evolution of hydrogen gas.

Example: Lithium (Li), Sodium (Na), etc.

Ionization enthalpy: Ionization enthalpy is defined as the amount of energy required to lose an electron from a neutral gaseous atom.

Ionization enthalpy of alkali metal:

• For the alkali metal, the valence shell contains one electron and the atomic size of alkali metals is the largest as compared to other elements in their respective periods.
• So, due to their large size and their valence shell electronic configuration, alkali metal easily loses its electron to attain noble gas configuration.
• Therefore, the ionization enthalpy of alkali metal is low.

Hence, alkali metals have low ionization enthalpy.