Paramagnetic and diamagnetic describe how a substance responds to a magnetic field, and the difference comes down to one thing: whether or not the atoms or molecules have unpaired electrons. Paramagnetic materials are pulled toward a magnet because they contain unpaired electrons. Diamagnetic materials are very slightly pushed away from a magnet because all their electrons are paired.
Why Unpaired Electrons Matter
Every electron spins, and that spin generates a tiny magnetic field. When two electrons share the same orbital, they spin in opposite directions, and their magnetic fields cancel each other out. That’s pairing. But when an electron sits alone in an orbital with no partner, its magnetic field has nothing to cancel it. The atom now acts like a miniature magnet with a permanent magnetic moment.
This is the core distinction. If a substance has one or more unpaired electrons per atom or molecule, it is paramagnetic. If every electron is paired, the substance is diamagnetic. All atoms exhibit a baseline diamagnetic response because an external magnetic field slightly alters how electrons orbit the nucleus, creating a tiny opposing field. But in paramagnetic substances, the much stronger effect of those unpaired electrons overwhelms the weak diamagnetic background.
How Paramagnetic Materials Behave
When you place a paramagnetic substance near a magnet, it gets pulled in. The unpaired electrons act as tiny magnets that tend to line up with the external field, creating a net attraction. This effect is real but modest. Unlike a refrigerator magnet (which is ferromagnetic), a paramagnetic material only responds when an external field is present. Remove the magnet, and the alignment disappears because thermal energy scrambles the electron orientations.
Temperature plays a direct role. At higher temperatures, thermal motion fights against magnetic alignment, so the paramagnetic attraction weakens. Cool the material down and the effect strengthens. This relationship is described by the Curie-Weiss law in physics, but the practical takeaway is simple: heat reduces paramagnetism.
Common paramagnetic substances include oxygen gas, aluminum, platinum, and most transition metal ions. Oxygen is a particularly famous example because its paramagnetism was a puzzle for chemists until molecular orbital theory explained it.
How Diamagnetic Materials Behave
Diamagnetic substances do the opposite: they are very slightly repelled by a magnetic field. With all electrons paired, there’s no permanent magnetic moment to align with the field. Instead, the external field induces a tiny opposing magnetic moment in the electron cloud. This repulsion is extremely weak, on the order of 100,000 times weaker than the attraction of a typical ferromagnet.
Water, copper, silver, gold, and bismuth are all diamagnetic. Their relative permeability (a measure of how they interact with magnetic fields) is just barely below 1.0. Water sits at 0.99999, copper at 0.99999, and even bismuth, one of the strongest diamagnetic elements, only reaches 0.99983. In everyday life, you’d never notice this repulsion without sensitive equipment.
One important point: every substance has a diamagnetic component. It’s a universal property of matter. Paramagnetic substances simply have an additional, stronger attraction that masks the underlying diamagnetic repulsion.
How to Predict Which One a Substance Is
For individual atoms or simple ions, you look at the electron configuration and count unpaired electrons. If there are any, the species is paramagnetic. If all electrons are paired, it’s diamagnetic.
Take chlorine as an example. Its electron configuration ends with 3s²3p⁵, which means five electrons occupy three p orbitals. Two orbitals are full (paired), one has a single electron. That one unpaired electron makes a chlorine atom paramagnetic. Noble gases like neon and argon, on the other hand, have completely filled subshells and are diamagnetic.
For molecules, you sometimes need molecular orbital theory rather than Lewis structures. Molecular oxygen (O₂) is the classic case. Its Lewis structure suggests all electrons are paired, which would make it diamagnetic. But experiments show that liquid oxygen is attracted to magnets. Molecular orbital theory resolves this by showing that O₂ has two unpaired electrons occupying separate antibonding orbitals. Nitrogen gas (N₂), by contrast, has all its electrons paired in its molecular orbital diagram and is diamagnetic.
Transition Metals and Spin States
Transition metals make this more interesting because their d orbitals can hold up to ten electrons, and the way those electrons fill in depends on the chemical environment. A bare iron ion might have four unpaired electrons, but surround it with different molecules (called ligands) and you can change that number dramatically.
This happens because ligands split the d orbitals into higher and lower energy groups. If the energy gap between those groups is large (caused by strong-field ligands like cyanide), electrons prefer to pair up in the lower orbitals rather than jump to the higher ones. This produces a “low spin” complex with fewer unpaired electrons, sometimes none at all, making it diamagnetic. If the gap is small (caused by weak-field ligands like chloride or iodide), electrons spread out across all d orbitals before pairing, giving a “high spin” complex with more unpaired electrons and stronger paramagnetism.
The spectrochemical series ranks ligands by how much they split the d orbitals. Strong-field ligands push toward low spin and potentially diamagnetic behavior. Weak-field ligands push toward high spin and paramagnetic behavior. This is why the same metal ion can be paramagnetic in one compound and diamagnetic in another.
Measuring Magnetic Properties in the Lab
Chemists measure paramagnetism and diamagnetism using devices that detect how a sample’s apparent weight changes in a magnetic field. In the Gouy method, a sample hangs from a balance between the poles of a magnet. With the field on, a paramagnetic sample appears heavier (it’s pulled toward the stronger part of the field) and a diamagnetic sample appears very slightly lighter (it’s pushed away). The difference in weight, with and without the field, reveals the magnetic susceptibility.
The Faraday balance works on the same principle but has a practical advantage: it needs only a small amount of sample and doesn’t require perfectly uniform packing. Both methods give a numerical value for magnetic susceptibility. Paramagnetic substances have positive susceptibility values. Diamagnetic substances have small negative values, typically on the order of negative 0.00001.
Real-World Applications
The most widely known application is in MRI (magnetic resonance imaging). Gadolinium, a strongly paramagnetic metal with seven unpaired electrons, is used as a contrast agent. When injected as part of a specially designed compound, it shortens the relaxation time of nearby water molecules, making certain tissues appear brighter on the scan. This helps radiologists spot tumors, inflammation, and blood vessel abnormalities.
Researchers have also developed diamagnetic contrast agents for a specialized version of MRI called magnetic resonance spectroscopic imaging. These agents work through a completely different mechanism, providing direct measurements of tissue acidity rather than relying on the water-relaxation effects that paramagnetic agents use.
Beyond medicine, magnetic susceptibility measurements help chemists determine the number of unpaired electrons in a new compound, which in turn reveals its electronic structure. This is especially valuable for transition metal complexes, where the spin state tells you about the bonding environment around the metal center.