1.6 Electron arrangements and ionisation energy

• Ionisation energy is the energy required to remove a mole of electrons from a mole of atoms in the gaseous state and is measured in kJmol-1.
• Ionisation energies generally increase across a period because the nuclear charge is increasing and this makes it more difficult to remove an electron.
• Going from magnesium to aluminium, the IE goes down, despite the increase in nuclear charge. This is due to the outer electron in aluminium being in the 3p orbital which is of a slightly higher energy than the 3s orbital. It therefore needs less energy to remove it
• There is a small drop between phosphorus and sulfur. In phosphorus, each of the three 3p orbitals contain just one electron, while in sulfur, one of the 3p orbitals must contain 2 electrons. The repulsion between these paired electrons makes it easier to remove one of them, despite the increase in nuclear charge.
• Ionisation energy generally decreases going down a group due to the outer electron being in a main level that gets further from the nucleus. The inner electron shielding outweighs the increase in nuclear charge.