There are two MO filling orders for homonuclear diatomics, and mixing them up is the most common JEE error. For molecules with Z <= 7 (Li2 through N2), s-p mixing pushes sigma(2p) above pi(2p). Order: sigma(2s) < sigma*(2s) < pi(2px) = pi(2py) < sigma(2pz) < pi*(2px) = pi*(2py) < sigma*(2pz). For Z >= 8 (O2 and F2), no significant s-p mixing, so sigma(2pz) drops below pi(2p). Order: sigma(2s) < sigma*(2s) < sigma(2pz) < pi(2px) = pi(2py) < pi*(2px) = pi*(2py) < sigma*(2pz). This switch explains why B2 is paramagnetic (electrons go into degenerate pi MOs) and why O2 is paramagnetic (unpaired electrons in pi* MOs).