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Question:
At a recent AP Chemistry conference the Molecular Orbital
diagram for carbon monoxide was shown. The Atomic orbital
diagram for oxygen was lower in energy then the carbon atom.
Someone suggested that O has lower energy because of the increased
nuclear attraction and the inverse square law predicts the
increase of  energy from like-charged electron repulsion. I
disagreed with the logic because oxygen, with more electrons,
would then be of higher energy then carbon. The instructor agreed
but could not answer my question with an alternative answer.
Can anyone help me with a better explanation?
 susan m buta

Answer:
In the "Molecular Orbital" approximation, one guesses
at the relative energies of the orbitals of the valence
electrons in order to guess how the molecular orbitals
will add together. The way that is usually successful is
adopt the following rule; electronegativity is a measure
of how strongly at atom within a molecule tends to attract
electrons to itself. Thus, the atom with the lower electronegativity
should have its atomic orbitals lower in energy than
the atom with higher electronegativity. This tends to
work pretty well overall.
In the case of NO, we are talking about a very small difference
in electronegativity...0.5 Pauling units. CO is a little
more "extreme" with a difference of 1.0. M
Robert Mullekin suggested that electronegativities can be
predicted from atomic properties. He suggested that
the electronegativity of an atom be given by half its ionization
energy (IE) minus electron affinity (EA):
X(Mulliken) = (IE - EA) / 2
So atoms that tend to pick up electrons easily (large, negative EA)
and hold onto them strongly (large positive IE) have a large
electronegativity. I
On the next "slide" let's look at some numbers for C and O.
 
-topper

 
Atom    Pauling EN       IE       EA         Mulliken EN
        (dimensionless) (kJ/mol)  (kJ/mol)     (kJ/mol)
 
C       2.5             1086     -122          604
N       3.0             1402        0          701
O       3.5             1314     -141          728
 
As you can see, the trends are the same in the two scales.
So Mulliken's formula really does predict how trends in
electronegativity go.  Now, notice that O > N > C. Second,
notice that the trends are not uniform in either IE or EA!
So, it is not sufficient to simply view the matter as due to
an increase in Z. In fact, this little "blip" occurs in
EVERY PERIOD in the periodic table...one way to interpret the
data is to note that the N atom has a half-filled shell,
an extremely stable situation due to a special property
(probably not discussed in your training thus far) called
Fermion exchange.
 
In any event, the usual way to get the ordering of valence shell
orbitals "right" for adding them up to form MOs is to use
electronegativities.  Since X_O > X_C, oxygen's levels should be
placed lower in energy than carbon's.

The most extreme example
of this effect might be HF, in which there is such a big difference
in Pauling electronegativity (about 2) that the 1s orbital of H
is similar in orbital energy to the 2px/2py/2pz orbital energies,
and thus bonding involves the combination/overlap of H(1s) with
F(2pz).
 
I hope this helps! This was an excellent question.
 
-topper