Q.What are interstitial compounds? Give the classification of alloys with examples.

Interstitial compounds are metal lattices with small non-metal atoms trapped in the gaps; alloys are classified by how the second component is incorporated and by whether iron is present.

Interstitial compounds: compounds formed when small atoms such as hydrogen, carbon, nitrogen, or boron become trapped in the interstitial voids/spaces of a transition metal's crystal lattice, without disrupting the parent metallic structure. They typically retain metallic conductivity/lustre, are very hard, chemically inert, and have high melting points. Examples: $TiC$, $Fe_3N$, $Mn_4N$, $VH_{0.56}$, $TiH_{1.7}$ (used in steel-making and as hard, refractory materials).

Classification of alloys:

1. Homogeneous alloys (solid solutions):
   • Substitutional alloys — atoms of similar size replace host metal atoms in the lattice, e.g. brass (Cu–Zn), bronze (Cu–Sn).
   • Interstitial alloys — small atoms occupy the interstitial voids of the host metal lattice, e.g. steel (Fe with interstitial C). …