Question

According to molecular orbital theory, there are_____ $\pi$ bonds in $C_2$ molecule.

Answer 1

Answer: 2

$C_2:\sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2{\sigma }^{\ast }2s^2\pi 2p_x^2=\pi 2p_y^2$
Therefore, according to molecular orbital theory, the orbitals involved in bonding are only $\pi$ orbitals $\left(\pi 2p_x,\pi 2p_y\right)$.
Since the last $4$ electrons are in the pi molecular orbitals, there are two $\pi$ bonds.