Paramagnetic vs Diamagnetic: Predicting Magnetism from Electrons

Hold a strong magnet near a stream of liquid oxygen and the liquid clings to the magnet’s poles. Try the same with liquid nitrogen, and it pours straight past. Neither is “a magnet” in the everyday sense, so why the difference? The answer lies in the electrons. Every substance responds to a magnetic field in some way, and whether it’s weakly attracted (paramagnetic) or weakly repelled (diamagnetic) can be predicted from its electron configuration.

Quick comparison

Feature	Paramagnetic	Diamagnetic
Unpaired electrons	one or more	none (all paired)
Behaviour in a magnetic field	weakly attracted into the field	very weakly repelled from the field
Strength of effect	small, but usually much larger than diamagnetism	very small
Temperature dependence	weaker at higher temperatures (thermal motion disrupts alignment)	essentially independent of temperature
Stays magnetised when field removed?	no	no
Examples	O₂, Fe³⁺, Cu²⁺, Mn²⁺, Al, Na atoms	N₂, H₂O, NaCl, Zn²⁺, Cu, most organic compounds

Where magnetism comes from

Each electron behaves like a tiny bar magnet because of its spin (and, to a lesser extent, its orbital motion around the nucleus).

• In a paired electron (two electrons in one orbital, ↑↓), the spins are opposite and their magnetic effects cancel.
• An unpaired electron (↑ on its own) has a magnetic effect that doesn’t cancel. The atom, ion or molecule then has a permanent magnetic moment.

In a magnetic field, these magnetic moments tend to line up with the field, so the substance is drawn into it: paramagnetism.

Diamagnetism is a separate effect present in all matter: an external magnetic field slightly alters the motion of all electrons, inducing a tiny opposing magnetic field. In substances with unpaired electrons, the paramagnetic attraction is much stronger and dominates. In substances with all electrons paired, diamagnetism is all that remains.

Predicting magnetic behaviour: the method

1. Write the electron configuration (for ions, remember transition metals lose 4s electrons first). See electron configurations of ions.
2. Draw the orbital boxes for any partly filled subshells, applying Hund’s rule.
3. Count unpaired electrons. See how to count unpaired electrons.
4. Any unpaired electrons → paramagnetic. None → diamagnetic.

Worked examples: atoms

Atom	Configuration	Unpaired	Prediction
He	1s²	0	diamagnetic
Li	[He] 2s¹	1	paramagnetic
C	[He] 2s² 2p²	2	paramagnetic
Ne	[He] 2s² 2p⁶	0	diamagnetic
Mg	[Ne] 3s²	0	diamagnetic
Zn	[Ar] 3d¹⁰ 4s²	0	diamagnetic

(These predictions apply to isolated atoms in the gas phase. In a solid metal, the electrons are shared across the whole lattice, and bulk magnetic behaviour can differ. Sodium metal, for example, is weakly paramagnetic because of its free electrons, while copper metal is diamagnetic.)

Worked examples: ions

Ion	Configuration	Unpaired	Prediction
Na⁺	[Ne]	0	diamagnetic
Sc³⁺	[Ar]	0	diamagnetic
Ti³⁺	[Ar] 3d¹	1	paramagnetic
Fe²⁺	[Ar] 3d⁶	4	paramagnetic
Fe³⁺	[Ar] 3d⁵	5	strongly paramagnetic
Cu⁺	[Ar] 3d¹⁰	0	diamagnetic
Cu²⁺	[Ar] 3d⁹	1	paramagnetic
Zn²⁺	[Ar] 3d¹⁰	0	diamagnetic

The more unpaired electrons, the stronger the paramagnetism. Fe³⁺ and Mn²⁺, with five unpaired d electrons each, are among the most strongly paramagnetic common ions. That’s why gadolinium(III) compounds, with seven unpaired 4f electrons, are used as contrast agents in MRI scans: they strongly affect the magnetic behaviour of nearby water molecules.

Note for complexes: in transition metal complexes, strongly interacting ligands can split the d orbitals enough to force electrons to pair. For example, Fe²⁺ in some complexes (such as [Fe(CN)₆]⁴⁻) has no unpaired electrons and is diamagnetic. Predictions from the free ion apply to “high-spin” cases.

Molecules: the oxygen puzzle

For many simple molecules, drawing a Lewis structure shows all electrons in pairs, suggesting diamagnetism. That’s correct for N₂, H₂O, CO₂ and CH₄.

But for oxygen, O₂, a Lewis structure with a double bond (O=O) shows all electrons paired, predicting diamagnetism. Experiment says otherwise: oxygen is paramagnetic. The explanation comes from molecular orbital theory, which shows that O₂ has two electrons in two separate, equal-energy antibonding orbitals, which by Hund’s rule remain unpaired. This was one of the early triumphs of molecular orbital theory over simpler bonding models. See Lewis dot structures.

The liquid oxygen demonstration is the dramatic proof: liquid O₂, which is pale blue, is held between the poles of a strong electromagnet, while liquid N₂ flows straight through.

Ferromagnetism: when paramagnetism gets organised

Iron, cobalt and nickel (and some alloys and compounds) show ferromagnetism, which is much stronger than ordinary paramagnetism:

• The atoms have unpaired electrons (like paramagnetic substances).
• In addition, neighbouring atoms’ magnetic moments align spontaneously in the same direction across regions called domains.
• When domains line up, the material becomes a permanent magnet.
• Above a certain temperature (the Curie temperature, 770 °C for iron), thermal motion destroys the alignment and the material becomes merely paramagnetic.

See magnetic elements.

Comparing the types

Type	Strength (relative)	Unpaired electrons?	Alignment
Diamagnetism	very weak repulsion	no	none (induced opposing field)
Paramagnetism	weak attraction	yes	random, until a field is applied
Ferromagnetism	very strong attraction; permanent magnets	yes	spontaneous, in domains

How magnetism is measured

Chemists measure how strongly a sample is attracted into a magnetic field using a Gouy balance or Evans balance (the sample appears to gain or lose mass when a magnet is applied), or more sensitive instruments. From the measurement, the magnetic moment is calculated, and from that, the number of unpaired electrons. For first-row transition metal ions, a useful approximation is the spin-only formula:

μ = √[n(n + 2)] Bohr magnetons, where n is the number of unpaired electrons.

For Fe³⁺ (n = 5): μ = √35 ≈ 5.9 Bohr magnetons, which matches experiment well.

Everyday and scientific uses

• MRI contrast agents (gadolinium compounds).
• Oxygen sensors: some analysers exploit O₂’s paramagnetism to measure oxygen concentration in gas mixtures, including in medical equipment.
• Magnetic levitation: strongly diamagnetic materials (and even living things, in extremely strong fields) can be levitated. A frog was famously levitated in a very powerful magnet in 1997.
• Identifying oxidation states of metal ions in new compounds, by measuring magnetic moments.

Practice questions

Predict whether each is paramagnetic or diamagnetic:

1. Ca²⁺
2. V³⁺ (V = [Ar] 3d³ 4s²)
3. Cr³⁺
4. Ni²⁺
5. N₂

Answers: (1) [Ar], diamagnetic; (2) [Ar] 3d², 2 unpaired, paramagnetic; (3) [Ar] 3d³, 3 unpaired, paramagnetic; (4) [Ar] 3d⁸, 2 unpaired, paramagnetic; (5) all electrons paired, diamagnetic.

A quick rule to remember: “paired means diamagnetic”.

Key takeaways

• Paramagnetic substances have unpaired electrons and are weakly attracted into a magnetic field.
• Diamagnetic substances have all electrons paired and are very weakly repelled.
• Predict magnetism by writing the configuration and counting unpaired electrons with Hund’s rule.
• Oxygen is paramagnetic, which Lewis structures can’t explain but molecular orbital theory can.
• Ferromagnetism (iron, cobalt, nickel) is a much stronger effect in which unpaired electrons align in domains.