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.