Ionisation Energies

The Ionisation Energy is the energy require to remove an electron from a gaseous atom or ion at stp.

There can be multiple ionisation energies, such as:

First Ionisation Energy: Energy required to remove an electron from a gaseuos atom to form a uni-positive ion.

$X \rightarrow X^{+}+e^-$

Second Ionisation Energy: Energy required to remove an electron from a gaseuos monopositive ion to form a di-positive ion.

$X^{+} \rightarrow X^{2+}+e^-$

Third Ionisation Energy: Energy required to remove an electron from a gaseuos dipositive ion to form a tri-positive ion.

$X^{2+} \rightarrow X^{3+}+e^-$

Trends Down a Group

The Ionisation Energy of elements down the group decreases. This is due to the fact that the outer electron is further away from the nucleus and therefore the nuclear pull will be lower.

Trends Along a Period

Along a period the atom decreases in size and therefore there will be an increase in the ionisation energy as the electron will be closer to the nucleus, with a higher effective nuclear pull on each electron.

Exceptions for Ionisations Energies Along a Period

There is a decrease in energy between Group 2 and Group 3 and between Group 5 and Group 6.

Group 2 and Group 3 difference

On going from Magnesium to Aluminium there is a full s orbital, resulting in a little bit of extra shielding and thus less pull from the nucleus.

Group 5 and Group 6 difference

On going from Phosphorus and Sulfur there is a change from a half-full p orbital to paired electrons in the p orbital. These paired electrons repel, making it easier to lose an electron.