IIT JEE , AIEEE Forum - Chemistry , Physical Chemistry - why we fill ?*2px or ?*2py first in O22- then in ?* 2pz.???-

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These MO are of two types loww energy mo wrt to AO are called BMO(bonding molecular orbits) and higher energy are called AMO(ANTIBONDING MOLECULAR ORBITALS)represented by * mark
sigma1s < sigma*1s < sigma 2s < sigma* 2s < sigma 2pz < pi 2px = pi 2py < pi* 2px = pi* 2py < sigma* 2pz this is true for Z >7
for z <7
sigma1s < sigma*1s < sigma 2s < sigma* 2s < pi 2px = pi 2py < sigma 2pz < pi* 2px = pi* 2py < sigma* 2pz

The basic reason is for this change is relative separation of 2s and 2p orbitals which increases along the peiod and switch occure at N2 (NITROGEN)
pi orbitals are less strongly bonded ie of higher energy as compared to sigma orbitals (as in F2 and O2)because their maximum overlaps occurs off axis, this relative weekness supports the change in energy order of orbitals

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