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Ionisation Energy

Chemistry: AQA A Level, topic 1.1.6

Equation for Ionisation Energy

The ionisation energy of an atom can be well defined by its ionisation equation.

The electrons

The electrons

  • Ionisation is the reaction where one electron is removed from an atom.
  • There will be one electron in the product of the equation.
  • So we will always have an e− term on the right-hand side of the equation.
The charges

The charges

  • We must understand the charges on the atom during the reaction.
  • During ionisation, we remove one electron from an atom.
  • As the negative electron is removed, the atom's charge will increase by 1.
    • E.g. the second ionisation of Na converts Na+ → Na2+.
The state symbols

The state symbols

  • Ionisation energies are always defined as being in the gas phase.
  • So atomic species of any charge, on either side of the equation, will have the state symbol (g).
  • We do not define the state of the electron - it has no state symbol.
Overall equation

Overall equation

  • Putting all these three features together, we gain the ionisation energy equation.
  • This is best shown by a few examples.
  • The first ionisation energy equation of Na:
    • Na(g) → e− + Na +(g)
  • The second ionisation energy equation of Mg:
    • Mg+(g) → e− + Mg 2+(g)

Ionisation Energies

A specific amount of energy is needed to remove an electron from an atom or ion.

First ionisation energy

First ionisation energy

  • The first ionisation energy is the energy required to remove an electron from every atom in a mole of atomic gas, to produce a mole of unipositive gaseous ions.
Second ionisation energy

Second ionisation energy

  • The second ionisation energy is the energy required to remove an electron from every ion in a mole of unipositive gaseous ions, to produce a mole of dipositive gaseous ions.
Third ionisation energy

Third ionisation energy

  • The third ionisation energy is the energy required to remove an electron from every ion in a mole of dipositive gaseous ions, to produce a mole of tripositive gaseous ions.
Fourth ionisation energy

Fourth ionisation energy

  • Do you see the pattern?
Nth ionisation energy

Nth ionisation energy

  • Overall we can form the equation shown here for the nth ionisation.
  • Note the value n, for the nth ionisation, is the value of the charge on the ion in the PRODUCTS.

Test yourself on Ionisation Energy

  • The ionisation of a species involves the removal of how many electrons?

  • What is the magnitude of the charge on the ion in the reactants of the nth ionisation equation?

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