Cue Column

• What causes variable oxidation states?
• What causes colour in ions?
• What causes paramagnetism?
• What makes them good catalysts?
• What condition is needed for d-d transition?

Notes Column

1. Variable Oxidation States Both (n-1)d and ns electrons participate in bonding. The energy difference between them is small, so varying numbers can be lost. Mn shows the widest range: +2 to +7. The highest oxidation state = group number (up to group 7 for Mn).

2. Coloured Ions Incomplete d-subshells allow d–d electronic transitions. Visible light is absorbed when an electron is promoted from a lower d-orbital (t2g) to a higher one (eg). The absorbed wavelength corresponds to $\Delta O$; the complementary colour is transmitted and observed. $Zn^{2+}$ (3$d^{10}$) and $Sc^{3+}$ (3d^0) are colourless because d–d transitions are impossible.

3. Paramagnetism Unpaired d-electrons generate a net magnetic moment. Magnetic moment: $\mu = \sqrt{n(n+2)}\text{ BM}$ where n = number of unpaired electrons. Diamagnetic ions (all paired) = n = 0.

4. Catalytic Activity Variable oxidation states allow the metal to act as an intermediary (accepting and donating electrons). Surface adsorption on the d-orbital lattice also lowers activation energy. Examples: Fe in Haber process, $V_{2}O_{5}$ in Contact process, $MnO_{2}$ in $KClO_{3}$ decomposition.

5. Coordination Compound Formation Small size, high charge density, and available empty d-orbitals allow transition metals to accept lone pairs from ligands.

Summary Box

Variable OS → catalysis and redox. Incomplete d → colour and paramagnetism. Small ionic size → coordination compounds.