Question

Why is $B_2$ paramagnetic while $C_2$ is diamagnetic?

Answer 1

Hint: The electron configuration of a substance can be used to identify its magnetic properties: The substance is paramagnetic if its electrons are unpaired, and diamagnetic if all of its electrons are paired.

Complete answer:
Paramagnetic molecules are those molecules in which unpaired electrons are present while diamagnetic molecules are those in which unpaired electrons are absent.
The molecular orbital electronic configuration of both $B_2$ and $C_2$ are:
$$B_2:{\left[\sigma 1s\right]}^2{\left[\sigma \ast 1s\right]}^2{\left[\sigma 2s\right]}^2{\left[\sigma \ast 2s\right]}^2{\left[\pi 2p_x\right]}^1{\left[\pi 2p_y\right]}^1$$
$$C_2:{\left[\sigma 1s\right]}^2{\left[\sigma \ast 1s\right]}^2{\left[\sigma 2s\right]}^2{\left[\sigma \ast 2s\right]}^2{\left[\pi 2p_x\right]}^2{\left[\pi 2p_y\right]}^2$$
As $B_2$ has two unpaired electrons, it is paramagnetic, while $C_2$ has no unpaired electrons, hence it is diamagnetic.
Hence, we can conclude that $B_2$ is paramagnetic while $C_2$ is diamagnetic.

Additional Information: Examples of Paramagnetic materials include magnesium, molybdenum, lithium, and tantalum. Examples of diamagnetic substances are bismuth, carbon, mercury, lead. The paramagnetic materials are attracted to a magnet only weakly. It indicates that under the influence of any applied magnetic field, paramagnetic materials are weakly attracted. These materials are unable to generate a magnetic field due to the paired electrons between the atoms. Any applied magnetic field repels or opposes diamagnetic materials.

Note:
Every substance in our environment has certain magnetic characteristics. In the presence of a magnetic field, different materials exhibit distinct properties. The electrons contained in the atoms or molecules give a substance its magnetic characteristics. Every electron in an atom acts as a little magnet. Electrons are also known as tiny current loops that retain their magnetic properties.