The set that contains atomic numbers of only transition elements, is:

Question

Canvas Classes · Accepted Answer

Transition elements have partially filled d-orbitals in ground state or in common ionic states.

Checking Option (2): 21, 25, 42, 72

| Z | Element | Config | Transition? |
|---|---------|--------|-------------|
| 21 | Sc | [Ar]  | Yes ✓ |
| 25 | Mn | [Ar]  | Yes ✓ |
| 42 | Mo | [Kr]  | Yes ✓ |
| 72 | Hf | [Xe]  | Yes ✓ |

Why others fail:
- 37 (Rb): s-block; 50 (Sn): p-block; 64 (Gd): f-block (lanthanoid)
- 9 (F), 17 (Cl): p-block halogens; 34 (Se): p-block; 38 (Sr): s-block
- 32 (Ge): p-block;

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