emmalav
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Why is the first ionisation energy of aluminium lower than the first ionisation energy of magnesium . I understand that this may be due to the fact that the electron in aluminium is in a 3p orbital whereas the outer electron in magnesium is in a 3s orbital which is of slightly lower energy than the 3p orbital meaning the outer electron would be more difficult to remove. However, I thought that nuclear charge increased across the group and this would mean that aluminium would have a higher first ionisation energy than magnesium. Just as for example the element with the highest ionisation energy in group 2 is neon ?
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charco
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(Original post by emmalav)
Why is the first ionisation energy of aluminium lower than the first ionisation energy of magnesium . I understand that this may be due to the fact that the electron in aluminium is in a 3p orbital whereas the outer electron in magnesium is in a 3s orbital which is of slightly lower energy than the 3p orbital meaning the outer electron would be more difficult to remove. However, I thought that nuclear charge increased across the group and this would mean that aluminium would have a higher first ionisation energy than magnesium. Just as for example the element with the highest ionisation energy in group 2 is neon ?
You are correct on all counts ...

There is a general trend to increased 1st ionisation energy crossing a period due to increased nuclear attraction ... BUT

There is a slight decrease from Be to B and from Mg to Al due to the next electron being in a 'p' sub-shell, which is more diffuse and higher in energy than an 's' subshell.

Check out this interactive about 1st ionisation energy ... click on the data points for more information.
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Fayadh56
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(Original post by emmalav)
Why is the first ionisation energy of aluminium lower than the first ionisation energy of magnesium . I understand that this may be due to the fact that the electron in aluminium is in a 3p orbital whereas the outer electron in magnesium is in a 3s orbital which is of slightly lower energy than the 3p orbital meaning the outer electron would be more difficult to remove. However, I thought that nuclear charge increased across the group and this would mean that aluminium would have a higher first ionisation energy than magnesium. Just as for example the element with the highest ionisation energy in group 2 is neon ?
First off the full electron configurations,
Mg: 1s2 2s2 2p6 3s2
Al: 1s2 2s2 2p6 3s2 3p1

For Al you remove the electron from the incomplete 3p1 subshell, which is less stable than the 3s2 subshell in Magnesium, so it is easier to remove the electron from Al 3p1 subshell, thus Al has a lower 1st Ionising Energy.
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