Question

Explain, why the valency of the helium atom is not 2 even when the Helium atom has 2 electrons in its valence shell?

Answer 1

Hint: Try to think about why elements exhibit valency and why do they form bonds. Try to think about what state most elements attain after bond formation. Figure out what is the maximum number of electrons that can fit in the valence shell of the Helium atom.

Complete step by step answer: We know that Helium is an element from the first period. The atomic number of Helium is 2 so it has 2 electrons in total. The number of shells in the first period elements will be 1. And the number of subshells will also be 1. This subshell is 1s subshell. 1s subshell can accommodate a maximum of 2 electrons. So, the electronic configuration of the Helium atom is $1{ s }^{ 2 }$.
Any element mainly forms bonds for attaining stable configuration. Stable configuration means that either fulfilled valence shell or empty valence shell. Alkali metals and alkaline earth metals have 1 and 2 electrons respectively in their valence shell so they generally tend to lose their valence shell electrons and attain a stable configuration. Whereas halogens and chalcogens are 1 and 2 electrons short of their fulfilled valence shell respectively so they try to attain these electrons and attain a stable configuration.
Helium has fulfilled valence shell configuration which is stable. Any element forms bonds to attain a stable configuration. But Helium already has a stable configuration. So, it is reluctant to form bonds though it has 2 electrons in the valence shell.

Note: We know that many elements form a covalent bond which is sharing of electrons instead of forming an ionic bond which is either losing or gaining of electrons because if they gain or lose electrons charge density will be very large and they will be highly unstable. In covalent bond Due to the sharing of electrons both will attain stable configuration without any unbalanced charge.