Ionisation energies

• Reason for drop in ionisation energy in group 5 and 6

  DUE TO ELECTRON REPULSION:

  • Electrons in group 5 are rmoved from singly-occupied orbital
  • Whereas in group 6, electrons are removed from an orbital with TWO electrons
  • These two electrons in grp 6 orbital cause REPULSION therefore it is is easier to remove e- from shared orbitals
• Reason for drop in IE from grps 2 and 3?

  DUE TO SUBSHELL STRUCTURE

  • Grp 2 = S orbital
  • Grp 3 = P orbital
  • P orbital has higher energy than s orbital in same shell therefore the electron is FURTHER from nucleus compared to outer electron in S orbital
  • Additional shielding provided by the s electrons
  • These factors override increase in nuclear charge -> THEREFORE Ionisation energy DROPS
• Ionisation energy definition

  energy needed to remove 1 mole of electron from 1 mole of gaseous atoms
• Is ionisation endo or exothermic?

  endothermic
• how to from equations for successive ionisation?

  • Elements must be gaseous (g)
  • Each time you remove electron, gains one more positive charge
  • Electron that is removed must be shown

  O+(g) -> O2+ + e-
• How does ionisation energy change as you go down a group?

  it decreases
• How does ionisation energy change as you go across a period?

  it increases
• The Factors Affecting Ionisation Energy are...

  • Nuclear Charge
  • Distance from Nucleus
  • Shielding
• What is shielding?

  As the number of electrons between the outer electrons and the nucleus increases, the outer electrons feel less attraction towards the nuclear charge. This lessening of the pull of the nucleus by inner shells of electrons is called shielding
• A high ionisation energy means there’s a high attraction between the electron and the nucleus.
• definition for the second ionisation energy

  The second ionisation energy is the energy needed to remove 1 electron from each ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions.
• What happens to ionisation energy when going down the group?

  • Down a Group the number of shells increases hence the outer shell

  electrons are further from the nucleus.

  • There is more shielding due to more inner shell electrons being present.

  • These two factors outweigh the increase in nuclear charge and so the

  attraction between the nucleus and the outer electrons decreases.