46. NCERT Exemplar XVII

1. Helium atom has $2$ electrons in its valence shell but its valency is not $2$, Explain.

 

Helium has only one shell (K shell), with a maximum number of electrons of $6$$2$$8$. Hence, the valency is zerothreeone.

  

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

 

The atomic number of the element X is $2$ and the atom of X contains fourtwothree electrons in it. These two electrons fill the K shell completely. Thus, the valency of X is zerothreefourtwo.