which forms interstitial compounds?

Options

A. `Fe`
B. `Co`
C. `Ni`
D. `All`

Explanation

To determine which elements form interstitial compounds, we need to understand the concept of interstitial compounds and the characteristics of the elements involved.

### Step-by-Step Solution:

1. **Understanding Interstitial Compounds:**
- Interstitial compounds are formed when small atoms (like hydrogen, carbon, nitrogen, oxygen, etc.) occupy the interstitial spaces (voids) in the crystal lattice of a metal.

2. **Characteristics of Interstitial Compounds:**
- These compounds typically have high melting and boiling points compared to the parent metals.
- They are generally hard and brittle.
- They tend to be chemically inert, meaning they do not easily participate in chemical reactions.

3. **Identifying the Elements:**
- The question provides us with three transition metals: Iron (Fe), Cobalt (Co), and Nickel (Ni).
- All these metals are known to form interstitial compounds with small atoms due to their crystal lattice structure.

4. **Conclusion:**
- Since all three transition metals (Fe, Co, and Ni) can form interstitial compounds by trapping small atoms within their crystal lattice, the correct answer is that all of them form interstitial compounds.

### Final Answer:
**All of the above (Fe, Co, Ni) form interstitial compounds.**

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