GATE 2025 CY – Question 13

Inorganic Chemistry · Transition Metals: Coordination chemistry – structure and isomerism, theories of bonding (VBT, CFT, and MOT). Metal-metal multiple bonds. Energy level diagrams in various crystal fields, CFSE, applications of CFT, Jahn-Teller distortion. Electronic spectra of transition metal complexes: spectroscopic term symbols, selection rules, Orgel and Tanabe-Sugano diagrams, nephelauxetic effect and Racah parameter, charge-transfer spectra. Magnetic properties of transition metal complexes. Ray-Dutt and Bailar twists, Reaction mechanisms: kinetic and thermodynamic stability, associative, dissociative substitution and redox reactions. · 1 mark · Multiple choice

Upon cooling from room temperature, the magnetic susceptibility of MnO slowly increases until 118 K, and then it decreases. This phenomenon is known as

ferromagnetism
paramagnetism
antiferromagnetism
ferrimagnetism

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Correct answer: (C) antiferromagnetism

Explanation

Magnetic ordering shows up in the temperature dependence of the susceptibility $\chi$.

**Paramagnet:** $\chi$ increases steadily on cooling (Curie law).
**Ferromagnet / ferrimagnet:** $\chi$ becomes very large below the Curie temperature and stays large.
**Antiferromagnet:** $\chi$ increases on cooling, goes through a **maximum at the Néel temperature**, and then **decreases** as the neighbouring spins align antiparallel and cancel.

MnO behaves exactly this way: the susceptibility rises on cooling until 118 K (its Néel temperature) and then falls. This is **antiferromagnetism** (option C).