Answer: 4f electrons.
- A 3d electrons
- B 4p electrons
- C 4f electrons
- D 5d electrons
Correct answer: C. 4f electrons
Explanation: The diffuse 4f orbitals shield the nuclear charge poorly, so the effective nuclear charge increases across the series and the size steadily decreases (lanthanoid contraction).
Aqueous solutions of transition metal ions show characteristic colours caused by electrons absorbing visible light to jump between split d-orbitals; the exact colour depends on the metal, its oxidation state, and surrounding ligands.
Concept context
Transition metals (d-block) and lanthanides/actinides (f-block). Known for coloured compounds, variable oxidation states, complex formation, and catalytic properties. Frequently tested in JEE and NEET.