Factors Affecting Ionisation Energy
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
Successive 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
- The successive ionisation energies of an element increase
- This is because once you have removed the outer electron from an atom, you have formed a positive ion
- Removing an electron from a positive ion is more difficult than from a neutral atom
- As more electrons are removed, the attractive forces increase due to decreasing shielding and an increase in the proton to electron ratio
- The increase in ionisation energy, however, is not constant and is dependent on the atom’s electronic configuration
- Taking calcium as an example:
Ionisation Energies of Calcium Table