i) What are interstial compounds? (1) ii) Transition metals show good catalytic property. Give any two reasons

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Q. i) What are interstial compounds?

(1)

ii) Transition metals show good catalytic property. Give any two reasons. (2)

Karnataka PUCKarnataka II PUC Board 2019Subjective· 3mImportance★★★★★

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Concept understanding — Interstitial Compounds

You are looking at a piece of steel. It feels hard, strong, and it doesn't rust easily. That steel is mostly iron, but it behaves nothing like pure, soft iron. What changed? Tiny carbon atoms slipped into the spaces between the iron atoms. That is the entire intuition behind interstitial compounds.

Think of a crowded train compartment. The seats are the metal atoms, packed in a regular lattice. Now imagine a very small child (a carbon, hydrogen, or nitrogen atom) squeezing into the narrow gap between the seats — not replacing anyone, just standing in the empty space. That child is an interstitial atom. The train (the metal lattice) is now harder to deform, heavier, and its properties have changed completely, even though the child didn't take a seat.

The Precise Statement

An interstitial compound is a compound formed when a small non-metal atom (typically hydrogen, boron, carbon, or nitrogen) occupies the interstitial voids (the gaps) in the crystal lattice of a transition metal, without significantly disrupting the metal's original lattice structure.

Important

The key idea: the metal atoms keep their original arrangement. The small atoms simply fit into the holes. This is not a normal chemical reaction where atoms swap places; it is an occupation of empty space.

Because the metal lattice is largely unchanged, these compounds often retain metallic properties like electrical conductivity and a metallic lustre. But because the small atoms wedge the metal atoms slightly apart and block slip planes, the material becomes dramatically harder and more brittle.

Why They Are Special (and Exam-Relevant)

Non-Stoichiometric: This is the most exam-critical point. Normal compounds have a fixed ratio (e.g., NaCl is always 1:1). Interstitial compounds rarely do. The number of small atoms that fit depends on how many voids are available, and that number varies. So you get formulas like $\ce{TiH_{1.7}}$ or $\ce{Fe_{3}C}$ (cementite) — the ratio is not a simple whole-number multiple of a valency. The formula is a range, not a fixed point.
Extreme Hardness: The small atoms lock the metal layers in place. This is why tungsten carbide ( $\ce{WC}$ ) is used in drill bits — it is nearly as hard as diamond.
High Melting Points: The metal-metal bonds are still strong, and the interstitial atoms add extra bonding. These compounds melt at very high temperatures.
Metallic Character: They conduct electricity and look like metals, because the metal lattice is intact. …

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(i) Interstitial compounds trap small atoms (H, C, N, B) in a transition-metal lattice's interstices. (ii) Transition metals catalyse via variable oxidation states (form intermediates) and surface adsorption of reactants.

(i) Interstitial compounds: Transition metals have gaps (interstices) in their lattices. Small non-metal atoms — hydrogen, carbon, nitrogen, boron — can lodge in these gaps to form interstitial compounds of variable (non-stoichiometric) composition, e.g. TiC, Mn $_4$ N, Fe $_3$ H. They are typically very hard, have high melting points and are chemically inert.

(ii) Catalytic property — two reasons:

Variable oxidation states: transition metals readily change oxidation state, so they can form unstable intermediate compounds with reactants and offer an alternative pathway of lower activation energy. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2020Set 56/2/11 markMCQ
$Q. The incorrect statement about interstitial compounds is (A) They are chemically reactive. (B) They are very hard. (C) They retain metallic conductivity. (D) They have high melting point.$
›Reveal solutionSolution
Interstitial compounds are formed when small atoms (C, N, H, B) occupy the interstitial voids in a transition metal lattice. They are very hard, have high melting points, and retain metallic conductivity — but they are chemically inert, not reactive. So the incorrect statement is (A).

Interstitial compounds are a fascinating class of materials. Think of them as a "host-guest" system: the host is a transition metal lattice (like iron, titanium, or tungsten), and the guest atoms — typically small non-metals like carbon, nitrogen, hydrogen, or boron — slip into the gaps (interstices) between the metal atoms. This isn't a normal chemical bond in the ionic or covalent sense; it's more like the guest atoms just occupy the empty spaces, distorting the metal lattice slightly.

Because the metal-metal bonding framework remains largely intact, these compounds often keep the metallic character — they conduct electricity and have a metallic luster. But the presence of the guest atoms makes the lattice much harder to deform, leading to extreme hardness and very high melting points (often rivaling the pure metal itself). However, the guest atoms are tightly locked inside; they don't easily react with outside chemicals. This makes interstitial compounds chemically inert, not reactive.

Let's check each option step by step.

Option (A): "They are chemically reactive."

This is the suspect. Interstitial compounds are famously unreactive. The guest atoms are shielded inside the metal lattice, and the strong metal–nonmetal interactions make them stable against attack by acids, bases, or other reagents. For example, titanium carbide (TiC) and tungsten carbide (WC) are used in cutting tools precisely because they resist chemical wear. So this statement is false.

Option (B): "They are very hard."

True. The inserted atoms block slip planes in the metal lattice, making it extremely resistant to deformation. Tungsten carbide (WC) is nearly as hard as diamond. This is a hallmark property. …
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Q.i) What are interstial compounds?

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