0On moving down in a group in p-block the higher valency becomes less stable and lower valencies become more stable. For example:
Stability : C+4 > Si+4 > Ge+4 > Sn+4 > Pb+4
C+2 < Si+2 < Ge+2 < Sn+2 < Pb+2
Carbon tetrabromide CBr4 is stable and exits while PbBr4 does not exits.
It is due increasing tendency of s-electron becoming inert in heavier elements (down the group). This reluctant of both s-electrons to take part in bonding is called the inert pair effect. This effect is especially seen in 13th and 14th group elements of the periodic table.
