Magnetic Properties

The magnetic behaviour of transition metal ions comes down to one question: how many unpaired electrons does the ion have? Get that count wrong, and you pick the wrong magnetic moment from the spin-only formula — a common source of lost marks.

Classification. Substances with unpaired electrons are paramagnetic (attracted into a magnetic field). Substances with all electrons paired are diamagnetic (weakly repelled). Among transition metals, paramagnetism dominates because partially filled d-orbitals readily hold unpaired electrons.

The spin-only formula. NCERT Class 12 Chemistry Chapter 4 (page 16) gives:

μ = √(n(n+2)) BM

where n = number of unpaired electrons. This approximation ignores orbital angular momentum contribution — valid for first-row d-block ions where crystal field quenching suppresses orbital contribution.

Counting unpaired electrons. Write the electronic configuration of the ion (not the atom). Remove electrons from 4s first, then from 3d. For example: Fe²⁺ is [Ar] 3d⁶ — four unpaired electrons in a free ion (high-spin). Fe³⁺ is [Ar] 3d⁵ — five unpaired electrons.

Watch-out. The trap is counting unpaired electrons from the neutral atom configuration rather than the ion configuration. Always strip 4s electrons first, then count d-orbital occupancy in the ionic state. A second common error: confusing high-spin and low-spin counts without being told the ligand field strength — in NEET, assume high-spin (weak field) unless the question explicitly states otherwise.