Concept understanding — Magnetic Properties of Transition Metals
Magnetic Properties of Transition Metals
The Intuition: Why Do Some Metals Get Pulled Into a Magnetic Field?
Imagine a tiny bar magnet. If you bring it near a strong magnet, it either gets pulled in or pushed away. Transition metals behave like collections of these tiny magnets — but the "magnet" inside each atom is the electron itself.
An electron is not just a charged particle; it also spins. That spin makes it behave like a microscopic current loop, which generates a magnetic field. In most atoms, electrons pair up with opposite spins, and their magnetic effects cancel out. But in transition metals, the d-orbitals are being filled one electron at a time. When an orbital has an unpaired electron, that electron's magnetic field is not cancelled. The atom as a whole becomes a tiny permanent magnet.
Now bring a bunch of these atomic magnets near an external magnetic field. They will try to align with it, and the material gets pulled into the field. That is paramagnetism. The more unpaired electrons an ion has, the stronger the pull.
Note
This is completely different from ferromagnetism (like iron nails). Ferromagnetism requires the atomic magnets to cooperate and align permanently with each other, even after the external field is removed. Paramagnetism is weaker and only exists while the external field is present.
The Precise Statement
The magnetic moment of a transition metal ion arises almost entirely from the spin of its unpaired electrons. The orbital contribution (the electron's motion around the nucleus) is usually "quenched" — suppressed by the electric field of surrounding atoms in a crystal or solution.
The magnitude of this spin-only magnetic moment is given by:
μ=n(n+2)Bohr magnetons
where n is the number of unpaired electrons.
μs=n(n+2)μB
This formula comes from quantum mechanics. Each unpaired electron has a spin quantum number s=1/2. For n unpaired electrons, the total spin quantum number S=n/2. The magnetic moment in Bohr magnetons is:
μ=gS(S+1)
where g≈2 for a free electron (the gyromagnetic ratio). Substituting S=n/2 gives:
μ=22n(2n+1)=n(n+2)
How to Use It: A Quick Table
Number of unpaired electrons (n)
μ (Bohr magnetons)
1
3≈1.73
2
8≈2.83
3
15≈3.87
4
24≈4.90
5
35≈5.92
Tip
Memorise the pattern: the values are roughly n+1 for small n, but the exact formula is what you need in exams. For n=5, the moment is nearly 6 — the maximum possible for a first-row transition metal ion.
Paramagnetism arises from the presence of unpaired electrons in an atom/ion. Transition (d-block) elements and their compounds very commonly have partially-filled d-orbitals (d1-d9 configurations), which frequently leave unpaired electrons — making paramagnetism a hallmark property of this block.
s-block elements (a) mostly have completely paired configurations in their common ions and are overwhelmingly diamagnetic. p-block elements (b) mostly achieve stable noble-gas-like paired configurations in their common ions. f-block elements (c) can also show paramagnetism (partially-filled f-orbitals), but it is the d-block that is classically cited for widespread, charac …