When we plot molecular orbital diagrams we use a linear combination of atomic orbitals.

Where can I find the energies of particular atomic orbitals?

In the picture below, the energies of 2s and 2p orbitals of Oxygen are lower than Carbon's 2s and 2p orbitals of Carbon, what is the logic behind this?

Molecular orbital diagram of CO2 from wikipedia

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    $\begingroup$ It has to do with the fact that oxygen has a higher effective nuclear charge. Thus, the oxygen orbitals are "closer" to the nucleus and therefor more stable. If I can get something to back it up, I will make an answer. $\endgroup$ – Ben Norris Mar 12 '13 at 21:42

Ben Norris's comment is correct.

An estimate of the atomic energies for orbitals may be gleaned from ionization energies, for example here:


Otherwise, the energies need to be calculated quantum mechanically.

  • $\begingroup$ This is just misleading. For elements greater in atomic number than lithium, the 2s orbital still has an ionization energy which increases with atomic number. The table that Lighthart linked is for outer shell ionization energies which is highly influenced by the shell structure of the atoms. $\endgroup$ – MaxW Oct 21 '15 at 21:24
  • $\begingroup$ It is fairly obvious the OP's question refers to valence electrons, rendering this comment moot at best. $\endgroup$ – Lighthart Oct 22 '15 at 20:39

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