The outermost electron in group 3 elements is being removed from the p-subshell which is higher in energy than the s-subshell so the p-electron takes less energy to remove.

The first ionisation energies of Al and Mg are represented below.

The outermost electron in group 6 elements is being removed from a paired p-orbital. There is greater repulsion between paired electrons in the same orbital than betwen unpaired electrons in different orbitals so the paired p-electron takes less energy to remove.

The first ionisation energies of S and P are represented below.

First ionisation energies generally increase across a period with 2 exceptions between groups 2-3 and groups 5-6.

For example, the trend in first ionisation energies of period 3 elements is shown below.

First ionisation energies decrease down a group.

For example, the first ionisation energies of group 2 elements decrease as we move down the group.

A large increase in successive ionisation energies suggests that the electron being removed is from a different shell, closer to the nucleus and with less shielding.

For example, the large increase between 7^th and 8^th successive ionisation energies of fluorine is explained in the figure below.

The first ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions.

For example, the first ionisation energy of lithium is represented by the equation: Li_(g) ➔ Li⁺_(g) + e^-

The 3 factors affecting ionisation energy are:

1. Nuclear charge
2. Atomic radius
3. Electron shielding

The greater the atomic radius, the greater the distance between the nucleus and the outer electrons, the weaker the electrostatic attraction, and the lower the ionisation energy.

The more protons there are in the nucleus, the greater the nuclear charge, the stronger the electrostatic attraction between the nucleus and the outer electrons, and the higher the ionisation energy.

Electron shielding is the repulsion between inner-shell electrons and outer-shell electrons which reduces the attraction between the nucleus and the outer electrons.

Successive ionisation energies are the energies required to remove successive electrons from an element, with each ionisation energy corresponding to the removal of one electron.

First ionisation energies increase down a group because:

• The outer electrons are further from the nucleus in successive periods, resulting in a weaker electrostatic attraction between the nucleus and the outer electrons.
• With each period, an additional electron shell is added, providing more shielding and further reducing the attraction between the nucleus and the outer electrons.
• The weaker attraction between the nucleus and outer electrons makes it easier to remove an electron, resulting in lower first ionisation energies down the group.

Mg⁺_(g) ➔ Mg²⁺_(g) + e^-