Ionisation Energies - anomaliesPeriod 2: 1. Be (group 2) and B (group 3). The decrease in ionisation energy from group 2 to group 3 is because the outer electron in boron has more energy since it is in the 2p orbital whilst the electron in beryllium is in the 2s orbital.2. N (group 5) and O (group 6). In oxygen, the first electron removed is one of the two paired electrons. This increases the electron-electron repulsion so less energy is needed to remove the electron.