Question

Y does value ionization enthalpy decrease down a group

Answer 1

Ionisation enthalpy is the energy needed to take out the valance electrons from the atom of an element and as we go down the group effective nuclear charge decrease on the Valence electron and ionisation enthalpy also decrease

Answer 2

Because down the group  atomic radius increases so distance from neucleus to outer electron increases  then less energy required to remove this last electron so ionisation enthalpy decreases down the group.                               If you are satisfied this answer then Mark as brilliant