Explain why transition metal ions are generally paramagnetic and coloured. Calculate the spin-only magnetic moment of Cu²⁺ (Z = 29).

1. 1 · Paramagnetism. Unpaired d electrons behave like tiny magnets, so the ions are attracted by a magnetic field.
2. 2 · Write the formula. μ = √(n(n+2)) BM, n = number of unpaired electrons.
3. 3 · Calculate for Cu²⁺. Cu²⁺ is [Ar] 3d⁹: n = 1, so μ = √(1 × 3) = √3 = 1.73 BM.
4. 4 · Explain colour. d-d transitions absorb visible light; the complementary colour is seen (Cu²⁺ looks blue).
5. 5 · Give the exception. d⁰ and d¹⁰ ions (Sc³⁺, Zn²⁺) are colourless and diamagnetic.
6. 6 · Close in one line. So both properties come from partly filled d orbitals.

Scoring tip: Write the ion's configuration before counting unpaired electrons; remove 4s electrons first.