Stability of Paired vs. Unpaired Electrons

[scope]

hi,

does anyone there know why two (antiparallel) electrons with net spin 0(paired electrons) are more stable than two unpaired electrons?

is it because the magnetic attraction between the magnetic dipoles of these 2 electrons?

this is a basic question therefore I need to understand it well!

 

[alxm]

does anyone there now why two (antiparallel) electrons with net spin 0 are more stable than an unpaired electron?

I don't see how those two states are directly comparable, since they can't be the same system. In the former case you'd have an even number of electrons, in the latter, an odd number of electrons. Either you have a singlet (all paired) or a triplet (two unpaired parallel spins), a quintet (four unpaired parallel spins) and so on, or you have a doublet (one unpaired), quartet (three unpaired parallel) and so on.

If the energy levels are degenerate, then the most stable state will normally be the one with the largest possible spin (Hund's rule). E.g. O₂, which has a triplet ground state because its HOMO is the doubly-degenerate pi*-orbitals.

The reason why most compounds are singlets is simply because you have two electrons per orbital (per the Pauli principle), and the orbitals aren't usually degenerate. But if you look at, say, transition-metal compounds, you often have degenerate d-levels, and subsequently end up with high-spin compounds quite often.

 

[scope]

ok, sorry I was just wondering about comparing 2 antiparallel electrons with net spin 0 and 2 electrons that are parallel. for example i have read this on wikipedia:

http://en.wikipedia.org/wiki/Oddo-Harkins_rule

" In elements with even atomic numbers, protons are paired, with each member of the pair offsetting the spin of the other, enhancing stability "

the same principle applies for electrons, so I was wondering what is the cause for this? I believe this is related to magnetic moment that lead to a magnetic attraction, or i am wrong?

 

[scope]

I don't see how those two states are directly comparable, since they can't be the same system. In the former case you'd have an even number of electrons, in the latter, an odd number of electrons. Either you have a singlet (all paired) or a triplet (two unpaired parallel spins), a quintet (four unpaired parallel spins) and so on, or you have a doublet (one unpaired), quartet (three unpaired parallel) and so on.

If the energy levels are degenerate, then the most stable state will normally be the one with the largest possible spin (Hund's rule). E.g. O₂, which has a triplet ground state because its HOMO is the doubly-degenerate pi*-orbitals.

The reason why most compounds are singlets is simply because you have two electrons per orbital (per the Pauli principle), and the orbitals aren't usually degenerate. But if you look at, say, transition-metal compounds, you often have degenerate d-levels, and subsequently end up with high-spin compounds quite often.

But this statement(Hunds rule) works well for unpaired electrons. But unpaired electrons are less stable than paired electrons, because unpaired electrons are more reactive. Do you agree with this, and what is the explanation for this enhanced stability?

 

[DrDu]

alxm's explanation is right. The direct magnetic interaction of two spins is tiny and does not play any role.

 

[scope]

ok, but why unpaired electrons are more reactive than paired electrons? if they are more reactive, this seems to mean they are less stable

 

[robert80]

It has nothing to do with stability I suppose. They just tend to have lower energy because the orbital space for 2 is much more limited (their position is more defined) and forming of the molecules is exotermic reaction for that reason.

 

[scope]

well a chemical reaction occurs because its products are more stable. further reactivity means further chemical reaction and therefore less stability. don't you agree?

 

[robert80]

Yes but it has nothing with the momentum. It has to do with that that the electron functions with placing 2 elelctrons in that space (chemical bonds) have lower energy. The magnetum momentum is not considered. the lowest energy for electron orbital function is with 2 electrons, that's a solution of wave functions. And the world is just so. Please do not correlate the enthalpy for chemical bond with magnetic spin.