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Ionisation question

Could someone help me with this question.

Which has a higher 1st ionisation energy; He or Li?
Why is this?

My guess is He because its outer electrons are closer to the nucleus. If this is correct, what other factors should I take into account.

Also, if you know, what is a spin-pair repulsion in the conext of ionisation energies?

Thanks, :smile:
It is higher for He, because it has a filled shell and is already very stable, Li has one outer electron, it's loss would leave Li with a full outer shell...so it's going to be relatively easy to remove it.

Spin-pair repulsion is the energy term associated with having two electrons in the same orbital - its better to have one electron in each orbital (3px1, 3py1, 3pz0) than having two electrons in one orbital and none in another (3px2, 3py0, 3pz0)
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Robbie10538
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Thanks for that :smile:

What about P and S? If they are on the same period how do you know which has a bigger ionisation energy?
They normally tell you which is bigger and you have to rationalise it :p:

Sulfur is slightly lower than Phosphorus due to the spin-pair repulsion energy...i.e. P is 3p3 and S is 3p4 (one double occupied orbital)

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