We can explain chemical anti-bonding just using the Pauli repulsion correct?Let's take He2.

2 atoms of He share 4 1s electrons and since the magnetic spin for electrons has 2 values there would be 2 electrons with the same wave function which would violate the Pauli exclusion principle.This makes He2 unstable and it disassociates in 2 He atoms

Am I correct?

  • $\begingroup$ The anti-bonding orbitals still obey the Pauli principle. You mixed up two different unrelated things. $\endgroup$ – Greg Nov 6 '19 at 3:38
  • $\begingroup$ No I say that the antibonding orbitals exist due to Pauli exclusion principle. $\endgroup$ – Mrs Chemistry Nov 6 '19 at 3:40
  • $\begingroup$ Let's put it this way: all orbitals except the lowest one are occupied due to Pauli exclusion principle. This applies to atoms as well. He2 is the only system where this is important for the molecule, but not for the atoms. $\endgroup$ – Ivan Neretin Nov 6 '19 at 6:32
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    $\begingroup$ Orbitals do not exist in a physical sense. You can argue, as @IvanNeretin that occupation of higher energy levels occurs because of the Pauli principle (without that all electrons would go to a single orbital), but those different orbitals (single-electron energy level) exist because they are solutions for the Schrodinger equation. $\endgroup$ – Greg Nov 6 '19 at 16:36
  • $\begingroup$ I know all of this ok?Just the existence of antibonding orbitals is due to the Pauli exclusion principle.In He2 there are 4 electrons in the same orbital ->2 of them must have the same wavefunction and this makes He2 unstable. $\endgroup$ – Mrs Chemistry Nov 6 '19 at 18:55

The reason we speak of orbitals as being exclusively inhabited by 2 (max) electrons (with opposing electron spin quantum number) is because electrons are fermions and therefore observe the exclusion principle. The exclusion principle constrains the allowed electron configurations, disallowing occupation of lower E orbitals by more than 2 electrons. In the case of $\ce{He2}$ it requires occupation of orbitals that raise the total E above the energy of the atoms at greater separation. So, in a word, the answer is yes.

The wikipedia entry on the Pauli exclusion principle alludes to its role:

It has been shown that the Pauli exclusion principle is responsible for the fact that ordinary bulk matter is stable and occupies volume. This suggestion was first made in 1931 by Paul Ehrenfest, who pointed out that the electrons of each atom cannot all fall into the lowest-energy orbital and must occupy successively larger shells. Atoms, therefore, occupy a volume and cannot be squeezed too closely together.[12]

I should add that dispersion interactions mean that there actually is a very weak attractive interaction between He atoms before the repulsive term "kicks in". See e.g. this wikipedia description of the London dispersion force, which has this illustration of the potential between two Ar atoms:

enter image description here

See also the table at the bottom of the wikipedia article, which explains that for the Ne dimer, dispersion contributes 100% of the total intermolecular interaction energy (I would modify that perhaps to attractive interaction).

  • $\begingroup$ Not just in He2 , but in Be2 as well and many more. $\endgroup$ – Mrs Chemistry Nov 6 '19 at 22:39

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