Question

Why does nitrogen combine with 4 hydrogen and not three hydrogen even though its valency is three?

Answer 1

The short answer is, although nitrogen has valency 3, it has 4 electron pairs around it, and it's the 4 electron pairs that allows it to form 4 bonds in the ammonium ion. A more detailed explanation is as follows:

I'll explain for the ammonium ion; the explanation for all other compounds in which nitrogen forms four bonds follows analogously:

First, consider the structure of NH3: the nitrogen has four electron pairs around it, three of which are shared pairs with hydrogen nuclei, and one of which is unshared. These electron pairs arrange themselves into an approximately tetrahedral shape, so that the hydrogens are on one side of the molecule and the nitrogen is on the other. This electron pair geometry exposes the unshared electron pair to the environment. If a hydrogen ion was in the environment, it would be attracted to the unshared electron pair and the attractive force between the hydrogen ion and the electrons would form a bond between the hydrogen ion and the nitrogen, so that the nitrogen has 4 hydrogens attached to it.

Answer 2

To maintain octet rule. If 3 electrons are paired octet rule will not be followed . So4 electrons