Ionisation Energy (AQA A-Level Chemistry): Revision Notes

1.1.6 Ionisation Energy

First Ionisation Energy

The first ionisation energy is defined as the amount of energy required to remove one mole of electrons from one mole of gaseous atoms, forming one mole of gaseous ions with a charge of +1. It is measured in kilojoules per mole (kJ/mol).

The equation for the first ionisation energy of an element $X$ is:

$$X(g) \rightarrow X^+(g) + e^-$$

Where:

• $X(g)$ represents a gaseous atom.
• $X^+(g)$ represents the resulting ion.
• $e^-$ is the electron that is removed.

Successive Ionisation Energies

Successive ionisation energies refer to the energy needed to remove more than one electron from an atom. As more electrons are removed, the ionisation energy increases because:

1. The electron being removed is closer to the nucleus.
2. There are fewer electrons to shield the attraction from the nucleus.

Key Equations for Successive Ionisation Energies

First Ionisation Energy

$$X(g) \rightarrow X^+(g) + e^-$$

Second Ionisation Energy

$$X^+(g) \rightarrow X^{2+}(g) + e^-$$

Third Ionisation Energy

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

These equations are useful for predicting how difficult it is to remove subsequent electrons and provide insights into the electronic structure of an atom.