• The ionisation energy (IE) of an element is the amount of energy required to remove one mole of electrons from one mole of atoms of an element in the gaseous state to form one mole of gaseous ions
• Ionisation energies are measured under standard conditions which are 298 K and 1 atm
• The units of IE are kilojoules per mole (kJ mol^-1)
• The first ionisation energy (IE₁) is the energy required to remove the first electron from an atom of an element
  • Eg. the first ionisation energy of gaseous calcium:

Ca(g) → Ca⁺ (g) + e^–          IE₁ = 590 kJ mol^-1

• The second ionisation energy (IE₂) is the energy required to remove the second electron from each ion in a mole of gaseous +1 ions
• The third ionisation energy (IE₃) is the energy required to remove the third electron from each ion in a mole of gaseous +2 ions
• The electrons from an atom can be continued to be removed until only the nucleus is left
• This sequence of ionisation energies is called successive ionisation energies

Successive ionisation energies of Beryllium table