1.1.7 Ionisation Energy: Trends & Evidence

• 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 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

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)
• Removal of the fourth electron is more difficult as the orbital is no longer full, and there is less spin-pair repulsion
• 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 shell and the small jumps are the change of 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 analyzing 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 electron 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
• The 3p electron 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