Ionisation Energy - Definitions
Ionisation energy
- 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 first ionisation energy (IE1) is the energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions
- E.g. the first ionisation energy of gaseous calcium:
Ca (g) → Ca+ (g) + e- IE1 = +590 kJ mol-1
Second and third ionisation energies of an element
- More than one electron can be removed form an atom and each time you remove an electron there is a successive ionisation energy
- These are called second, third ionisation energy and so on
- The second ionisation energy (IE2) is defined as
- is the energy required to remove one mole of electrons from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions
- And can be represented as can be represented as
X+ (g) → X2+ (g) + e-
OR (for calcium)
Ca+ (g) → Ca2+ (g) + e- IE2 = 1145 kJ mol-1
- The third ionisation energy (IE3) of an element can be represented as
X2+ (g) → X3+ (g) + e-
OR (for calcium)
Ca2+ (g) → Ca3+ (g) + e- IE3 = 4912 kJ mol-1