Which amongst the following compounds is a diamagnetic compound?

\(C_6H_6, CrO_2, MnO, MgFe_2O_4\)

\(C_6H_6\)

The correct answer is option 2. \(C_6H_6\).

A diamagnetic compound is one that is repelled by a magnetic field. This repulsion occurs because all the electrons in the molecule are paired, resulting in no net magnetic moment. Let's analyze each option to see if it meets this criteria:

1. \(C_6H_6\) (Benzene): This molecule has all its electrons paired in a delocalized aromatic ring system. Therefore, it is diamagnetic.

2. \(CrO_2\) (Chromium(IV) oxide): \(CrO_2\) is ferromgnetic. Ferromagnetic substances are those that are attracted by a magnetic field and show permanent magnetism even in the absence of a magnetic field. Ferromagnetism rises due to the spontaneous alignment of magnetic momentum in the same direction as shown below

\(CrO_2\) has \(3d\) orbital which overlap to form band. In \(CrO_2\) the band is very narrow so it shows ferromagnetism.

3. \(MnO\) (Manganese(II) oxide): Manganese in \(MnO\) has a \(+2\) oxidation state, meaning it has lost two d electrons. However, it still has five d electrons remaining, which can potentially be unpaired. Therefore, \(MnO\) is not diamagnetic and exhibits weak paramagnetism due to the presence of some unpaired electrons.

4. \(MgFe_2O_4\) (Magnesium Ferrite): This compound contains both \(Mg^{2+}\) and \(Fe^{3+}\) ions. \(Mg^{2+}\) has no d electrons, making it diamagnetic. \(Fe^{3+}\) has three unpaired d electrons, making it paramagnetic. However, in \(MgFe_2O_4\), the magnetic moments of these ions partially cancel each other out, resulting in a very weak net paramagnetism. Therefore, \(MgFe_2O_4\) can be considered practically diamagnetic due to its very weak magnetic response.