Ionisation Energy: Trends & Evidence (AQA A Level Chemistry): Revision Note

Ionisation Energies: Equations

• The second ionisation energy (IE₂) is the energy required to remove the second mole of electrons from each +1 ion in a mole of gaseous +1 ions, to form one mole of +2 ions

• The third ionisation energy (IE₃) is the energy required to remove the third mole of electrons from each +2 ion in a mole of gaseous +2 ions, to form one mole of +3 ions

• And so on...

• The electrons from an atom can be removed until only the nucleus is left

• This sequence of ionisation energies is called the successive ionisation energies

Successive Ionisation Energies of Beryllium Table

Successive Ionisation Energies

Successive ionisation energies of an element

• 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

• The first electron removed has a low IE₁ as it is easily removed from the atom due to the spin-pair repulsion of the electrons in the 4s orbital

• The second electron is more difficult to remove than the first electron, as there is no spin-pair repulsion

• The third electron is much more difficult to remove than the second one, corresponding to the fact that the third electron is in a principal quantum shell which is closer to the nucleus (3p)

• The third and fourth electrons both come from the 3p subshell, so the rise between them is gradual (same subshell)

• The graph shows there is a large increase in successive ionisation energy as the electrons are being removed from an increasingly positive ion

• The big jumps on the graph show the change of the shell, and the small jumps are the change of the subshell 

• Successive ionisation data can be used to:

  • Predict or confirm the simple electronic configuration of elements

  • Confirm the number of electrons in the outer shell of an element

  • Deduce the Group an element belongs to in the Periodic Table

• By analysing where the large jumps appear and the number of electrons removed when these large jumps occur, the electron configuration of an atom can be determined

• Na, Mg, and Al will be used as examples to deduce the electronic configuration and positions of elements in the Periodic Table using their successive ionisation energies

Successive Ionisation Energies Table

Sodium

• For sodium, there is a huge jump from the first to the second ionisation energy, indicating that it is much easier to remove the first electron than the second

• Therefore, the first electron to be removed must be the last in the valence shell, thus Na belongs to group I

• The large jump corresponds to moving from the 3s to the full 2p subshell

  Na       1s² 2s² 2p⁶ 3s¹

Magnesium

• There is a huge increase from the second to the third ionisation energy, indicating that it is far easier to remove the first two electrons than the third

• Therefore, the valence shell must contain only two electrons, indicating that magnesium belongs to group II

• The large jump corresponds to moving from the 3s to the full 2p subshell

  Mg       1s² 2s² 2p⁶ 3s²

Aluminium

• There is a huge increase from the third to the fourth ionisation energy, indicating that it is far easier to remove the first three electrons than the fourth

  • Al is therefore in group III (group 13)

• The 3p electrons and 3s electrons are relatively easy to remove compared with the 2p electrons, which are located closer to the nucleus and experience greater nuclear charge 

• The large jump corresponds to moving from the third shell to the second shell

  Al         1s² 2s² 2p⁶ 3s² 3p¹

Related topics

• What is Ionisation Energy?

• Electron Configuration