Question

Carbon and silicon are mainly tetravalent but Ge, Sn and Pb show di-valency. Explain.

Answer 1

Valency of an element is the characteristics of an individual element i.e. it is the combining power with another element in nature which can complement with respect to its valency with the prior one.
In short, the valency of an element is related to how many electrons are present in the outer shell of the same.

Complete answer:
Let us know about the elements mentioned above i.e. C, Si, Ge, Sn and Pb in detail;
The elements mentioned are the elements of the same group i.e. 14. So, they should have equal valency due to the same electronic configuration. They are p-block elements having electronic configuration as $n{{s}^{2}}n{{p}^{2}}$ . So technically, they all must be divalent.
But, only Ge, Sn and Pb are known as divalent due to presence of inert pair effect. This is because of the poor shielding effect of d and f electrons which is factually not present in C and Si (clear absence of d orbitals).
The inner s electrons would not have inertness in case of carbon and silicon due to absence of inert pair effect. In case of others, the s electrons will have inertness which would be then responsible for bringing down the oxidation state to 2.
Thus, C and Si would be tetravalent but other atoms with large atomic numbers will be divalent.

Note:
Do note that the elements in the same group of the modern periodic table have the same valency. But, in this case there is presence of inert effect which will be responsible to cross this normal behaviour due to presence of d and f electrons in higher atoms.