Diamagnetic vs paramagnetic behavior reflects whether a substance has a net magnetic moment from its electrons. Electron pairing and molecular orbital occupancy control the sign of magnetic susceptibility and the direction of the response in an applied magnetic field.
Magnetic response and electron pairing
Electrons possess intrinsic spin and therefore a magnetic moment. A pair of electrons in the same orbital has opposite spins, so their magnetic moments cancel nearly completely. Unpaired electrons leave a net magnetic moment that can align with an external field.
Diamagnetic substances: all electrons paired; net magnetic moment ≈ 0; weakly repelled by a magnetic field; magnetic susceptibility χ < 0.
Paramagnetic substances: one or more unpaired electrons; net magnetic moment > 0; weakly attracted by a magnetic field; magnetic susceptibility χ > 0.
Visualization of diamagnetic vs paramagnetic response
Magnetic susceptibility and temperature behavior
Magnetic susceptibility χ measures the degree of magnetization in an applied field. Diamagnetism corresponds to negative susceptibility, while paramagnetism corresponds to positive susceptibility. The paramagnetic response typically increases as temperature decreases, because thermal motion disrupts alignment.
The qualitative temperature trend is often summarized by Curie-type behavior for many paramagnets:
\[ \chi \propto \frac{1}{T} \]Diamagnetic susceptibility is comparatively small and weakly temperature dependent, because it arises from induced electronic motion rather than alignment of permanent moments.
Electron-count criteria in atoms and ions
Electron configurations predict the presence or absence of unpaired electrons. Closed-shell configurations (noble-gas-like) are diamagnetic. Partially filled subshells frequently produce paramagnetism because Hund’s rule favors unpaired electrons in degenerate orbitals.
| Property | Diamagnetic | Paramagnetic |
|---|---|---|
| Unpaired electrons | 0 | ≥ 1 |
| Susceptibility χ | Negative (χ < 0) | Positive (χ > 0) |
| Field response | Weak repulsion | Weak attraction |
| Temperature dependence | Small | Often decreases with increasing T |
| Microscopic origin | Induced electronic currents opposing B | Partial alignment of permanent magnetic moments |
Molecular orbital perspective and the O₂ exception
Molecular magnetism depends on the occupancy of molecular orbitals, not on counting electrons in isolated atoms. The classic general-chemistry example is O2, which is paramagnetic because its highest-occupied orbitals include two unpaired electrons in π* antibonding orbitals.
In contrast, many isoelectronic diatomic molecules such as N2 and CO have all electrons paired in their molecular orbitals and are diamagnetic.
Representative examples
| Species | Magnetic behavior | Electron-structure reason |
|---|---|---|
| Ne | Diamagnetic | Closed shell; all electrons paired |
| Zn(s), Zn2+ | Diamagnetic | Filled 3d subshell (3d10); no unpaired electrons |
| O2(g) | Paramagnetic | Two unpaired electrons in π* molecular orbitals |
| NO(g) | Paramagnetic | Odd-electron molecule; one unpaired electron |
| Fe3+(aq) | Paramagnetic | 3d5 often contains multiple unpaired electrons |
| Cu+(aq) vs Cu2+(aq) | Cu+: diamagnetic; Cu2+: paramagnetic | 3d10 (paired) vs 3d9 (one unpaired) |
Magnetic moment and unpaired-electron count
For many coordination compounds and ions where orbital contributions are modest, a useful estimate for the magnetic moment uses the number of unpaired electrons \(n\) (spin-only approximation). The value is commonly reported in Bohr magnetons (BM).
\[ \mu_{\text{so}} \approx \sqrt{n(n+2)}\ \text{BM} \]The estimate supports the main qualitative distinction: \(n = 0\) corresponds to diamagnetism, while \(n \ge 1\) corresponds to paramagnetism. Deviations occur when orbital angular momentum contributes significantly.
Common pitfalls and nearby terms
- Ferromagnetism: strong bulk attraction from domain alignment; distinct from ordinary paramagnetism even though unpaired electrons are still present at the atomic level.
- Closed-shell ions: cations and anions can be diamagnetic even when the neutral atom is not, because electron loss or gain changes subshell occupancy.
- Molecular electron counting: atomic electron configurations do not replace molecular orbital occupancy for diatomic molecules and many radicals.
Summary
Diamagnetic vs paramagnetic behavior is determined by electron pairing. Diamagnetic substances contain only paired electrons and show χ < 0 with weak repulsion, while paramagnetic substances contain unpaired electrons and show χ > 0 with weak attraction; electron configuration and molecular orbital occupancy provide the structural basis for the prediction.