Question

what happens to effective nuclear charge as you go down the group and why?

Answer 1

Well, effective nuclear charge normally decreases as you go down a group because the electrons get farther and farther away from the nucleus. Electron shielding effect also plays a role in reducing effective nuclear charge as it detracts/"shields" the farthest electrons from the pull of the nucleus. And the intervening electrons increase greatly while going down the group also the atomic size increases down the group

That's why effective nuclear charge decreases down the group.