Transition metals in which vacent spaces are occupied by small atoms such as hydrogen, carbon etc. are called `"….................."`

Answer

**Step-by-Step Solution:**

1. **Understanding the Concept of Interstitial Compounds:**
- Transition metals can form compounds where small atoms (like hydrogen, carbon, or nitrogen) occupy the vacant spaces (or voids) in their crystal lattice structure.

2. **Definition of Interstitial Compounds:**
- These compounds are specifically known as "interstitial compounds." They are formed when smaller atoms fit into the interstitial spaces of a metal lattice.

3. **Examples of Interstitial Compounds:**
- Common examples include titanium carbide (TiC), where carbon occupies the voids in the titanium structure, and titanium hydride (TiH1.7), where hydrogen occupies the voids in titanium.

4. **Properties of Interstitial Compounds:**
- Interstitial compounds have unique properties:
- They are often non-stoichiometric, meaning their composition can vary.
- They tend to be very hard and have high melting points.
- They retain metallic conductivity, similar to the parent metal.

5. **Conclusion:**
- Therefore, the transition metals in which vacant spaces are occupied by small atoms such as hydrogen, carbon, etc., are called "interstitial compounds."

**Final Answer:**
Transition metals in which vacant spaces are occupied by small atoms such as hydrogen, carbon, etc., are called **interstitial compounds**.

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