Question

Why interstitial hydrides have a lower density than the parent metal.

Answer 1

The term interstitial compound, or interstitial alloy, is used to describe a compound that is formed when an atom of sufficiently small radius sits in an interstitial “hole” in a metal lattice.

The term interstitial compound, or interstitial alloy, is used to describe a compound that is formed when an atom of sufficiently small radius sits in an interstitial “hole” in a metal lattice.

Answer 2

First of all i would like to tell you
What an interstitial compound means ?
The term interstitial compound, or interstitial alloy, is used to describe a compound that is formed when an atom of sufficiently small radius sits in an interstitial “hole” in a metal lattice.
The empty spaces between the molecules of the pure metal are the spaces where a molecule of relatively small radius can be filled.
So,density is always increased in interstitial compounds not decreased when compared with its parent pure metal