Assertion (A): Combining capacity of an element is valency.

Reason (R) : Elements lose or gain electrons to attain the nearest stable inert gas configuration.

Generally inert gases or zero group elements have high stability due to their octet electron configuration (${\text{ns}}^{\text{2}}{\text{np}}^{\text{6}}$). 

The remaining all elements also try to get octet configuration by combining with other atoms of elements.

So, the combining capacity of an atom of element either by lose or gain of electrons is called “valency”. 

Ex: For N valency is 3 and for F valency is 1.