Group 1 Elements

The ionization energy is the energy required to remove an electron from an atom in its gaseous state. For alkali metals, the first ionization enthalpy decreases as you move down the group. This is because the atomic size increases, and the outermost electron is farther from the nucleus, thus less strongly attracted. Lithium (Li), being at the top of the alkali metal group, has the highest first ionization enthalpy because its outermost electron is closer to the nucleus and more tightly bound.