Question

the first ionization enthalpy of group 13 elements is less than those of the corresponding group 2 elements explain​

Answer 1

first of all, you should understand the actual concept of ionisation energy. ionisation energy is the amount of energy required to remove an electron from an isolated gaseous atom. Now can you think, how this energy will be maximum and how will be minimum ?
of course , you can.

if outermost orbit of an element is complete its octet or outermost orbit contains half filled orbital then, element has higher ionisation energy due to remove an electron, element loses its stability.

general electronic configuration of group 13 is ns² np¹ [ neither half filled nor full filled p-orbital ]

general configuration of group 2 is ns² [ full filled s-orbital ]

here p- orbital of group 13 has only one electron so, it can easily remove from its isolated form. while s- orbital of group has two electrons( full filled orbital). element is in stable from, so higher amount of energy required to remove an electron. that's why ionisation energy of group 2 is higher than ionisation energy of group 13 .