Question

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

Answer 1

For $B_2\left({10}_{e^{-}}\right)$, the MO configuration is $\left(\sigma 1s{)}^2\right({\sigma }^{\star }1s{)}^2\left(\sigma 2s{)}^2\right({\sigma }^{\star }2s{)}^2(\pi 2p_x^1\approx \pi 2p_y^1)$

Due to the presence of unpaired electrons $(2p_x^1=\pi 2p_y^1)B_2$ shows paramagnetism.

For $C_2\left(12e^{-}\right)$, the MO configuration is $\left(\sigma 1s{)}^2\right({\sigma }^{\star }1s{)}^2\left(\sigma 2s{)}^2\right({\sigma }^{\star }2s{)}^2(\pi 2p^{2_x}\approx .\pi 2p^{2_y})$

No unpaired electrons are there in $C_2(\pi 2p_x^2=\pi 2p_y^2)$, hence it shows diamagnetism.