[theRomanticPast]

Why is the octet rule a rule?

I understand that energetically it makes no sense to go to the next shell and you would then find atoms with electrons fewer than that but why is it the most stable at closed shell configuration? Why can't you just partially fill the p-orbitals?

I understand that as electrons are paired up in orbitals the angular momentum is cancelled which lowers the energy. But this energy should be quite small. Also, if this is the reason for the octet rule then angular momenta is crucial to chemical bonding (the very driving force in fact) which is not covered anywhere in textbooks.

Cant find any explanation for this can someone help me with this?

[mjc123]

Try this thread: http://www.chemicalforums.com/index.php?topic=77729.msg283639#msg283639

[theRomanticPast]

That thred does not go into the mechanism behind why the octet rule (or maybe more general the closed-shell rule) is true.

It only says that the lattice energy is minimized when the valence orbitals are filled (paired up). Which is the very reason why the octet rule is a rule in the first place.

My question, again, is: please give me a plausible underlying reason why precisly closed shell atoms/compounds are more stable than others?

My thoughts:

-Is it the neutralization of the angular momenta that causes it?

-Or is it just a coincidence with these compounds that the MOs which are formed exactly matches a more stable configuration (more bonding orbitals than anti-bonding everytime for closed-shell configuration)?


Or is it something else? Please don't tell me "it has a lower energy with closed-shell" because that is not an explanation.

[Enthalpy]

Pairing is not a matter of cancellation, as you guessed. For instance in O₂, two electrons stay unpaired on two orbitals, as this is more favourable by 94kJ/mol than pairing both electrons on a single orbital.

It's more that, when two atoms interact because they're close to an other, half of the new orbitals (bonding) offer a lower energy and the other half (antibonding) a higher one. Because two paired electrons (opposite spins) can fill the same orbital, they may both fill the bonding one, so the result of the atoms' interaction is to lower the energy.

This depends on how many electrons are brought by the atoms, as compared with how many bonding orbitals are created. For N₂ the numbers fit perfectly, so the bonds are strong and the molecule inert. O₂ has 2 electrons more which find only antibonding orbitals, so the molecule is more reactive, and F₂ even more.
https://en.wikipedia.org/wiki/Triplet_oxygen

s² p⁶ configurations seem especially favourable, with the next d shell always at a much higher energy, but I ignore why. Noble gases (except He) have this configuration, while other shells keep reactive when full. Maybe this relates with the octet rule.