Ionisation energies increase across a
period. This is because the nuclear charge is increasing as there are more protons, but the amount of shielding is the same or similar so the atomic radius decreases across a period.
Shielding refers to the repulsion of the electron to be removed by the electrons in the inner
shells.
From
group 2 to 3 however, the ionisation energy decreases. This is because elements in group 3 have their outer electron in the p
orbital, which is of slightly higher energy than the s orbital outer electrons of group 2 elements. The p orbital electron in group 3 needs less energy to remove it.
The ionisation energy decreases from group 5 to 6. This is because the electrons in group 6 are paired and the repulsion makes it easier to remove.
| 1s 2s 2px 2py 2pz |
| nitrogen | ↿⇂ | ↿⇂ | ↿ | ↿ | ↿ |
| 1s 2s 2px 2py 2pz |
| oxygen | ↿⇂ | ↿⇂ | ↿⇂ | ↿ | ↿ |
Ionisation energy decreases down a group. This is because there are more shells so there is more shielding from inner shell electrons and the electron to be removed is farther away from the nucleus. This outweighs the increased attraction due to the increased nuclear charge.
