I don't understand why antibonding orbitals exist or why electrons fill them if they are counterproductive of the goal of bonding? I understand that bonding means the wave functions constructively interfere which attracts the two atoms together and makes them stable....but if antibonding means the phases are out of phase, the atoms are not attracted to one another, and those orbitals are even higher in energy than those of isolated atoms, then why does it even occur? Does antibonding have to occur in order for bonding to occur?
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I think this is a way to look at it.
Say the energy of isolated an electron in isolated atom is 2. in anti bonding orbital is 3 and bonding orbital is 1.
Now we are considering a case where 3 of them combine:
initially, energy of system is 2*3=6
say they combine and two go to bonding and another to anti bonding
now energy would be (2*1)+3=5
Thus we see a net decrease in energy.
in fact this is the reason why a species is feasible only if number of bonding electrons is more than number of anti bonding electrons.
(For the electron which goes to the anti bonding orbital, energy increases but for the entire system, there will still be a decrease in energy if the resultant molecule has to exist)
Say the energy of isolated an electron in isolated atom is 2. in anti bonding orbital is 3 and bonding orbital is 1.
Now we are considering a case where 3 of them combine:
initially, energy of system is 2*3=6
say they combine and two go to bonding and another to anti bonding
now energy would be (2*1)+3=5
Thus we see a net decrease in energy.
in fact this is the reason why a species is feasible only if number of bonding electrons is more than number of anti bonding electrons.
(For the electron which goes to the anti bonding orbital, energy increases but for the entire system, there will still be a decrease in energy if the resultant molecule has to exist)
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It is, if you like, a result of the 'laws' of quantum mechanics that when two atomic orbitals combine they produce two molecular orbitals - one bonding and one anti-bonding - as a result of the two 'allowable' ways of combining those two original atomic orbitals.
The anti-bonding orbital is always of higher energy, so, in a simple molecule like the hydrogen molecule the two electrons will occupy the bonding orbital, giving a bond order of one (corresponding to a 'single' bond)
Helium atoms won't combine because the four electrons would mean two in the bonding and two in the anti-bonding - giving a bond order of zero, i.e. no bond. But if you make the (He2)2+ ion, the two higher energy anti-bonding electrons are removed and you're back to a bond order of one (a single bond) - and this species has been identified. It's odd little facts like this that can really only be explained by MO theory.
Hope this helps.
The anti-bonding orbital is always of higher energy, so, in a simple molecule like the hydrogen molecule the two electrons will occupy the bonding orbital, giving a bond order of one (corresponding to a 'single' bond)
Helium atoms won't combine because the four electrons would mean two in the bonding and two in the anti-bonding - giving a bond order of zero, i.e. no bond. But if you make the (He2)2+ ion, the two higher energy anti-bonding electrons are removed and you're back to a bond order of one (a single bond) - and this species has been identified. It's odd little facts like this that can really only be explained by MO theory.
Hope this helps.