mot(molecular orbital theory) is different for above 14 electron 

Dear student,

Electronic configuration of molecules containing 14 or less electrons :

• This difference is dependent on the decreasing value of 's-p bonding interactions' i.e. bond interactions between 2s orbital of one atom and 2p of another atom.
• Now on moving from left to right in the periodic table i.e. from $L{i}_2 to F_2$, the energy difference between 2s and 2p orbitals increases due to increase in nuclear charge and lower shielding of 2s electrons by 2p electrons.
• Due to which, increasing s-p interactions on $\sigma 2p_z$ becomes less important.
• Hence, after $N_2$ molecule the energy of $\sigma 2p_z$ molecular orbital becomes lower than that of $\pi 2p_x and \pi 2p_y$.

  Regards