Ionisation energies generally increase across a period, because the nuclear charge is increasing (due to more protons), making it more difficult to remove an electron.
Exceptions:
Period 3:
- From magnesium to aluminium the ionisation energy decreases, because the outer electron in aluminium is in a 3p orbital, which is of a slightly higher energy than the 3s orbital, it therefore needs less energy to remove it.
- From phosphorus to sulfur the ionisation energy decreases. In phosphorus, each of the three 3p orbitals contains just one electron, while in sulfur, one of the 3p orbitals must contain two electrons. The repulsion between these paired electrons makes it easier to remove one of them.
Both these cases, which go against the expected trend, are evidence that confirms the existence of s- and p- sublevels.
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