Atomic structure
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I am confused because we have been told that the dip in 1st ionisation energies between nitrogen and oxygen is due to the way that the p sub-shell is filled, but this contradicts the other things we have been told about electrons being more stable in pairs. The 1st IE of oxygen is supposed to be lower because there are two electrons in the p subshell X space, where as in nitrogen there is only 1 electron in this space. Do electrons exist in pairs or on their own?
Ulex says
It is important to bear in mind that electrons and atoms do not have preferences as if they were living things. When circumstances change, electrons and atoms react to this change in whatever way is most energetically favourable.
In the nitrogen atom there are three p electrons. Since electrons are negatively-charged and would normally repel each other, it seems reasonable that the three electrons would achieve a state of lowest potential energy if they occupied separate p orbitals. In oxygen, there is an extra p electron which cannot go into an orbital on its own because all three p orbitals are singly occupied. So where does it go? It could go into a higher orbital, but it achieves a lower energy state by pairing with one of the other 2p electrons.
The only thing that makes this possible seems to be a property which an electron has, called ‘spin’. No-one is absolutely sure what spin means. Niels Bohr considered an electron to be a spinning charged object which generates a magnetic field. If two electrons in the same orbital spin in opposite directions, their magnetic fields exert some attraction for each other. We don’t accept Bohr’s model of the atom any more, but there does seem to be some property which is still called ‘spin’ which results in this attraction, and which partly offsets the repulsion between the negative charges. It is nevertheless easier to remove this paired electron than it is to remove one which occupies an orbital alone.
Sometimes this spin-pairing effect is strong enough to provide a more stable alternative where there are two possibilities as, for example in the copper atom where instead of having 3d9 4s2, the atom has 3d10 4s1.
Like all theories of sub-atomic behaviour, this is all a gross over-simplification. An eminent theoretician used to say in his lectures that if you wake up one morning with a clear understanding of how electrons behave, you are wrong! The human mind seems to be able to cope with things which are average-sized but has great difficulty with the very small (sub-atomic particles) and the very large (galaxies). Our best efforts are pretty pathetic really!
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updated: 26 January 2005
