MO Z<8 vs Z>/=8


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Gabrielle Schwab 1G
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Joined: Fri Sep 25, 2015 3:00 am

MO Z<8 vs Z>/=8

Postby Gabrielle Schwab 1G » Thu Oct 29, 2015 1:33 pm

Could someone please explain why diatomic molecules with Z greater than or equal to 8 have p sigma bonds that are lower in energy than their p pi bonds but it is the opposite for molecules with Z less than 8.

Chem_Mod
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Re: MO Z<8 vs Z>/=8

Postby Chem_Mod » Fri Oct 30, 2015 12:10 pm

I will cover this in my review session today (Friday).

Shaye Busse 3B
Posts: 20
Joined: Fri Sep 25, 2015 3:00 am

Re: MO Z<8 vs Z>/=8

Postby Shaye Busse 3B » Fri Oct 30, 2015 10:26 pm

The reason for this has to due with the electron repulsion between the s sigma * orbital and the next highest orbital. In molecules with an atom that is Z<8, the molecular orbital structure that accounts for the lowest possible energy state has the p pi orbitals in a lower position than the p sigma orbital. The opposite can be said about molecules that are strictly z>/=8 as the molecular orbital structure that accounts for the lowest possible energy state places the p sigma orbital lower than the p pi orbitals. The electron repulsion in molecules with an atom Z<8 between the s sigma * orbital and the p sigma orbital is great enough to where the p sigma and p pi orbitals swap positions along the molecular orbital structure. The resulting molecular orbital structure with p pi orbitals in a lower energy state than the p sigma energy state creates a molecule in an overall lower energy state.


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