Ionisation energy is the energy required to remove a mole of electrons from a mole of isolated gaseous atoms or ions.
Ionisation energies generally increase across a period because the nuclear charge is increasing and it's more difficult to remove an electron.
- Aluminium has one electron in 3p orbital which is of slightly higher energy than the 3s orbital and therefore it's easier to remove an electron.
- In sulfur, one of the 3p orbitals has a pair of electrons that repel each other and it's also easier to remove one.
Going down the group, the nuclear charge increases. However, the actual positive charge is less than the full nuclear charge. This is because of the effect of the inner electrons shielding the nuclear charge. The electrons in the outer shell are also further from the nucleus.
This results in decreased ionisation energy as we go down the group.