`C` and `Si` are always tetravalent , but Ge, Sn and Pb show divalency Give reason .

Explanation

Among the elements of group 14, carbon does not have d- or f-electrons . Therefore , it does not show inert pair effect. Consequently, it shows an oxidation state of`+4`due to the presence of two electrons in the s- and two electrons in the p-orbital of the valence shell. In contrast , all other elements from Ge to Pb contain either dor both d- and f-electron increases, the inert pair effect becomes more and more prominent. In other words, as we move dow the group from Ge to Pb, the stability of +2 oxidation state increases while that of `+4` oxidation state decreases. Therefore , the tendency of Ge, Sn and Pb to exhibit +2 oxidation state increases with increasing atomic number in group 14.