Question

why is the ionization enthalpy of hydrogen much higher than that of alkali metals

Answer 1

Dear Student,
Ionisation enthalpy is the minimum amount of energy required to remove the most loosely held electron from the valence shell of an isolated gaseous atom to form cation.
Removal of valence electron becomes easier with increase in atomic size due decrease in attractive force of nucleus on that electron. In other words, ionisation energy is inversely proportional to atomic size in a group.
Due to its smaller atomic size than alkali metals, ionisation energy or enthalpy of hydrogen is much higher than that of alkali metals.
Regards