Many chemical properties of elements depend on how readily the atom gives up an electron. The energy required to remove an electron from an atom in the gas phase can be measured. This quantity is referred to as the ionisation energy (enthalpy).
The ionisation energy for a particular atom depends on how far the outermost electron is from the nucleus.
Electrons that are in higher energy states due to being well separated from the nucleus are easier to remove, and the element has a lower ionisation energy.
Electrons that are in lower energy states due to being closer to the nucleus are more difficult to remove, and the element has a higher ionisation energy.
The first ionisation energy (enthalpy) for element (E) is the enthalpy change for the reaction below.
E(g)
E+(g) + e–
This reaction is endothermic for all elements because energy is required to separate charge. Ionisation energies are given for a few atoms below.
E(g)
E+(g) + e–This reaction is endothermic for all elements because energy is required to separate charge. Ionisation energies are given for a few atoms below.
| element symbol | ionisation enthalpy/ kJ mol–1 |
| Na | 494 |
| F | 1680 |
| Cu | 745 |
The ionisation energy for a particular atom depends on how far the outermost electron is from the nucleus.