Chapter 4 — The d- and f-Block Elements

Class 12 Chemistry Chapter 4 covers the d- and f-block elements — the transition metals (Groups 3–12) and the inner transition metals (lanthanoids and actinoids) — explaining their electronic configurations, variable oxidation states, magnetic behaviour, coloured ions, catalytic properties, and key compounds such as K₂Cr₂O₇ and KMnO₄.

The d-block (Groups 3–12) and f-block elements occupy the middle and bottom sections of the periodic table. Transition metals are defined as metals with an incomplete d subshell in the neutral atom or its ions; zinc, cadmium and mercury are excluded. These elements show variable oxidation states, paramagnetism, formation of coloured ions and complex compounds, catalytic activity, interstitial compound formation, and alloy formation. The f-block comprises two series: lanthanoids (4f, Ce–Lu) and actinoids (5f, Th–Lr). Lanthanoid contraction causes the second and third d-series to have nearly identical radii. Important compounds include K₂Cr₂O₇ and KMnO₄, both strong oxidising agents with wide industrial and analytical uses.

1. 01Transition metals are in Groups 3–12; their d orbitals are progressively filled across four series (3d, 4d, 5d, 6d). Zn, Cd and Hg are not true transition metals because their d orbitals are completely filled in ground state and common oxidation states.
2. 02Variable oxidation states arise from incomplete d-orbital filling; manganese shows the widest range (+2 to +7). Oxidation states in d-block elements differ by one unit, unlike p-block elements where they differ by two.
3. 03Transition metals are paramagnetic due to unpaired d electrons; magnetic moment is calculated using the spin-only formula µ = √(n(n+2)) BM, where n is the number of unpaired electrons.
4. 04Coloured ions result from d–d electronic transitions when energy absorbed falls in the visible region; ions with completely filled or empty d orbitals (e.g., Sc³⁺, Zn²⁺) are colourless.
5. 05Lanthanoid contraction — the steady decrease in atomic and ionic radii from La to Lu — causes the 5d elements (e.g., Hf, 159 pm) to have almost identical radii to their 4d counterparts (e.g., Zr, 160 pm), making their separation in nature difficult.
6. 06Actinoids (Th–Lr) exhibit a wider range of oxidation states than lanthanoids because the 5f, 6d and 7s energy levels are comparable; all actinoids are radioactive, with heavier members having very short half-lives.