Diamagnetism — Core Principles

Core Principles

Diamagnetism is a fundamental magnetic property present in all materials, though it is often weak and masked by other magnetic effects. It originates from the orbital motion of electrons within atoms.

When an external magnetic field is applied, it induces a change in the electron's orbital motion, which, by Lenz's Law, creates an induced magnetic moment that *opposes* the external field. This results in diamagnetic materials being weakly repelled by magnets.

Key characteristics include a small, negative magnetic susceptibility ($chi < 0$), relative permeability slightly less than 1 ($mu_r < 1$), and independence from temperature. The induced magnetization is temporary and disappears when the external field is removed.

Examples include water, copper, bismuth, and noble gases. Understanding diamagnetism is crucial for distinguishing it from paramagnetism and ferromagnetism in NEET.

Important Differences

vs Paramagnetism

Aspect	This Topic	Paramagnetism
Origin	Diamagnetism: Induced magnetic moment due to orbital electron motion opposing external field (Lenz's Law). No permanent atomic dipoles.	Paramagnetism: Alignment of pre-existing permanent atomic magnetic dipoles (due to unpaired electron spins) with the external field.
Behavior in External Field	Diamagnetism: Weakly repelled. Moves from stronger to weaker field regions.	Paramagnetism: Weakly attracted. Moves from weaker to stronger field regions.
Magnetic Susceptibility ($chi$)	Diamagnetism: Small and negative ($chi < 0$, e.g., $-10^{-5}$ to $-10^{-9}$).	Paramagnetism: Small and positive ($chi > 0$, e.g., $10^{-3}$ to $10^{-5}$).
Relative Permeability ($mu_r$)	Diamagnetism: Slightly less than 1 ($mu_r < 1$).	Paramagnetism: Slightly greater than 1 ($mu_r > 1$).
Temperature Dependence	Diamagnetism: Largely independent of temperature.	Paramagnetism: Strongly dependent on temperature (decreases with increasing temperature, follows Curie's Law: $chi propto 1/T$).
Electron Configuration	Diamagnetism: Typically atoms/ions with all electron spins paired (closed shells).	Paramagnetism: Atoms/ions with unpaired electron spins.
Examples	Diamagnetism: Water, Copper, Bismuth, Gold, Noble gases, Organic compounds.	Paramagnetism: Aluminum, Platinum, Oxygen, Sodium, Copper Chloride.

The fundamental distinction between diamagnetism and paramagnetism lies in the origin of their magnetic response. Diamagnetism is an induced effect, arising from the modification of electron orbital motion by an external field, always opposing it. Paramagnetism, conversely, stems from the alignment of pre-existing permanent atomic magnetic dipoles (due to unpaired electron spins) with the external field. This leads to diamagnetic materials being weakly repelled with negative susceptibility, while paramagnetic materials are weakly attracted with positive susceptibility. Crucially, diamagnetism is temperature-independent, whereas paramagnetism diminishes with increasing temperature.