7.2 Ionisation energies

• Ionisation energy refers to the energy needed to remove electrons
• The value of the ionisation energy depends on the attraction between the electron being lost and the nucleus
• Three factors affecting ionisation energy:

  • atomic radius = greater atomic radius, smaller attraction
  • nuclear charge = greater nuclear charge, greater attraction
  • electron shielding = greater shielding, smaller attraction
• First ionisation energy is the energy required to remove one electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions
• The value of successive ionisation energy increases with ionisation number

  • as each electron is lost, there is the same number of protons attracting fewer electrons
  • electrons are drawn closer to the nucleus increasing attraction
• Any large increase between successive ionisation energies shows that the next electron is being removed from a different shell
• First ionisation energy decreases down a group

  • atomic radius increases
  • more shielding
  • less attraction between nucleus and outer electrons
• First ionisation energy increases across a period

  • nuclear charge increases
  • shielding stays relatively the same
  • more attraction between nucleus and outer electrons, decreasing atomic radius
• Fall between Group 2 and 3

  • In group 3, electron is lost from the p orbital not the s orbital
  • lost more easily to has smaller ionisation energy
• Fall between 5 and 6

  • In group 5, all 3 p orbitals contain one electron
  • In group 6, one pair of electrons in p orbital
  • paired electrons repel each other and is more easily lost