Chemistry - Complete s-block Question with Solution | TestHub

Ionization enthalpy decreases down a group in the periodic table due to an increase in atomic size and shielding effect.

Among alkali metals (Group 1), Lithium ($\text{Li}$) is at the top of the group and has the smallest atomic size, hence it has the highest first ionization enthalpy.

Among alkaline earth metals (Group 2), Beryllium ($\text{Be}$) is at the top of the group and has the smallest atomic size, hence it has the highest first ionization enthalpy.

Therefore, the elements with the highest first ionization enthalpy are $\text{Li}$ and $\text{Be}$ respectively.

Answer: $\text{Li}$ and $\text{Be}$