There is a general increase in
ionisation energy across a
period.
There is a sharp decrease from the end of one period to the beginning of the next.
The atom radius decreases across a period as the number of protons (nuclear charge) increases. There is similar
shielding as the outer electrons are in the same quantum
shell but the increase in nuclear charge leads to a greater nuclear attraction so more energy is required to remove the electron.
Period 2 ionisation energies kJmol
-1:
| Li | Be | B | C | N | O | F | Ne |
| 520.2 | 899.5 | 800.6 | 1086.5 | 1402.3 | 1313.9 | 1681.0 | 2080.7 |
