Anong forup `14` elements `C` and `Si` have noble gas caore heneath the valence shell electrons wherwseas in `Ga, Sn` and `Pb` fully filled lesser shielding `d` and (or) f-orbitals are present in-=between the noble gas coare and valence shell electron . As a result inerst pair effect is exhibited by Ga, Sn and Pb and not ny `C` and `Si`. Hence `C` and (are alwyas tentravelent but `1` Ga Sn and Pb are always divlent in nature .
