stability

hmmmmmmmm......

think in a simple way. if u r discussing of metal groups , then certainly refers to the s- block elements.
now , in gr -1 and 2 , the electropositive character increases down the group ,
while in other groups , the electronegative character increases down the group.

thus, as in other groups , as the electronegative character increases down the group , the bonding electrons tend to move towards the central atom down the group. thus, bonded pair - bonded pair repulsion increases down the group , decreasing the stability.

also , many of these other groups hv lone pairs ( gr-1 and 2 don't hv ). thus the lone pair-bonded pair repulsion also increases down the group. this adds to decreasing stability.