Question

Which elements are called transition elements? Explain: (i) Transition metals generally form coloured compounds, (ii) Transition metals or ions and many of their compounds show paramagnetic behaviour.

Hint:

Transition elements differ from s- and p-block elements because their d-orbitals allow variable oxidation states, colour (d–d transitions), and magnetism (unpaired electrons).

Answer 1

Transition elements:
- Transition elements are those d-block elements which have partially filled d-orbitals in their atoms or in one of their common oxidation states.
- General definition: Elements of groups 3 to 12 (except Zn, Cd, Hg which have completely filled d-orbitals).
(i) Coloured compounds:
- Transition metal ions form coloured salts/complexes due to d–d electronic transitions.
- In the presence of ligands, the d-orbitals split into two sets of different energies (Crystal Field Splitting).
- Absorption of light promotes an electron from lower to higher d-orbital; the complementary colour is observed.
Examples: $\mathrm{[Cu(H_2O)_6]^{2+}}$ (blue), $\mathrm{[Ti(H_2O)_6]^{3+}}$ (purple).
(ii) Paramagnetic behaviour:
- Transition metals and their ions often contain unpaired d-electrons.
- Unpaired spins produce magnetic moments, giving paramagnetism.
- Paramagnetism is measured in Bohr magnetons (BM).
Examples: Fe$^{2+}$ ($d^6$, 4 unpaired e$^-$), Mn$^{2+}$ ($d^5$, 5 unpaired e$^-$).
\[ \boxed{\text{Thus, coloured compounds and paramagnetism are due to partially filled d-orbitals.}} \]