First 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 first ionisation energy decreases from group 5 to 6. This is because the electrons in group 6 are paired and the repulsion makes its easier to remove.
| 1s 2s 2px 2py 2pz |
| nitrogen | ↿⇂ | ↿⇂ | ↿ | ↿ | ↿ |
| 1s 2s 2px 2py 2pz |
| oxygen | ↿⇂ | ↿⇂ | ↿⇂ | ↿ | ↿ |
