Although there is a general trend in first ionisation energies across a period, and the ionisation energy increases across the period, except from group 2 to group 3.
Boron has a lower ionisation energy than beryllium because outer electron of beryllium is in the 2s sub-shell whilst the outer electron of Boron is in the 2p sub-shell.
The 2p electron is easier to remove than the 2s electron because the 2p sub-shell has a higher energy than the 2s sub-shell.
Period 2 ionisation energies kJmol-1:
Boron has a lower ionisation energy than beryllium because outer electron of beryllium is in the 2s sub-shell whilst the outer electron of Boron is in the 2p sub-shell.
The 2p electron is easier to remove than the 2s electron because the 2p sub-shell has a higher energy than the 2s sub-shell.
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 |