Interstitial Compounds

Interstitial compounds are formed when small non-metal atoms such as hydrogen, carbon, boron, or nitrogen are trapped in the interstitial sites, the empty spaces between the larger metal atoms in the crystal lattice of a transition metal. Because the small atoms fit into existing gaps rather than replacing metal atoms, these compounds are often non-stoichiometric and retain metallic conductivity while becoming much harder and having higher melting points than the pure metal. Steel, an interstitial compound of iron and carbon, is a familiar example. The option of replacing metal atoms describes substitutional alloys, not interstitial compounds. The option of forming ionic bonds only is incorrect, since the bonding is largely metallic with some covalent character. The option of dissolving the metal misdescribes the process. The nature of interstitial compounds is a standard NCERT d-block topic. Working through the logic step by step, rather than memorising the result, makes it clear why lattice spaces governs the behaviour seen here. Such questions reward conceptual clarity, since a student who truly grasps interstitial compounds can solve many superficially different variants with the same approach. Plausibility check: the increased hardness and high melting point of steel compared with pure iron directly illustrate the strengthening effect of interstitial carbon, confirming the answer.