Ionisation energy

Across a period, first ionisation energy generally increases due to increasing nuclear charge with similar shielding, leading to a stronger attraction between the nucleus and outer electrons. The atomic radius also decreases, further increasing the attraction.

Down a group, first ionisation energy generally decreases due to increasing atomic radius and increased shielding by inner shells. This results in a weaker attraction between the nucleus and outer electrons.

Successive ionisation energies increase because each electron is being removed from an increasingly positive ion. This requires more energy to overcome the greater attraction between the nucleus and the remaining electrons.

The main factors are: (1) Nuclear charge (2) Atomic/ionic radius (3) Shielding by inner shells/sub-shells (4) Spin-pair repulsion. These factors affect the strength of attraction between the nucleus and the valence electrons.

Large jumps in successive ionisation energies indicate the removal of an electron from a new electron shell, closer to the nucleus. By counting the number of electrons removed before each large jump, the number of electrons in each shell can be determined, revealing the electronic configuration.

Spin-pair repulsion occurs when two electrons occupy the same orbital. This repulsion makes it slightly easier to remove one of these electrons, resulting in a lower ionisation energy than expected compared to an electron in a singly occupied orbital.

Aluminium's outer electron is in a 3p orbital, which is higher in energy and therefore further from the nucleus compared to Magnesium's 3s electrons. The 3p electron is also slightly more shielded. This makes the removal of the 3p electron easier, resulting in a lower first ionisation energy for Aluminium.