An element $X$ has a mass number 4 and atomic number 2 .
Question:

An element $X$ has a mass number 4 and atomic number 2 . Write the valency of this element.

Solution:

The element $X$ with mass number 4 and atomic number 2 is helium $(H e)$. It has 2 electrons (maximum possible) in its only shell which is K-shell. Therefore, the valency of the element is zero.