Ionisation Energy Definition & Equations
- Ionisation is the process by which an electron is removed from an atom or a molecule
Process of Ionisation
In the first ionisation of sodium, the single outer electron is lost
- The ionisation energy (IE) of an element is the amount of energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous ions
- Ionisation energies are measured under standard conditions which are 298 K and 101 kPa
- The units of IE are kilojoules per mole (kJ mol-1)
- The values for ionisation energies are always positive as this is an endothermic process
- This is because energy is required to break the force of attraction between the electron and the central positive nucleus
First ionisation energy
- The first ionisation energy (IE1) is the energy required to remove one mole of electrons from one mole of atoms of an element to form one mole of 1+ ions
- E.g. the first ionisation energy of gaseous calcium:
Ca (g) → Ca+ (g) + e- IE1 = +590 kJ mol-1
Second ionisation energy
- The second ionisation energy (IE) of an element is the amount of energy required to remove one mole of electrons from one mole of gaseous ions of an element to form one mole of gaseous 2+ ions
- E.g. the second ionisation energy of gaseous calcium:
Ca+ (g) → Ca2+ (g) + e- IE2 = +1145.4 kJ mol-1
Exam Tip
- It is a common mistake to write ionisation energies in more than one step
- For example; Ca (g) → Ca2+ (g) + 2e-.
- Ionisation energies should take place one step at a time.
- Students often miss out the gaseous state symbol in the equation and also in the definition, so make sure you include this.
