periodicity : ionisation energies and atomic radii

• trends across a period

  • the number of protons in the nucleus increases, so there is a stronger attraction on the electrons
  • electrons are added to the same shell, so the outer shell is drawn inwards slightly
  • there is the same number of inner shells, so electron shielding will hardly change
• Decreases between group 2 to 13

  • small decrease in first ionisation energy between them due to group 13 elements have their outermost electron in a p-orbital, whereas group 2 elements have their s-orbital
  • p-orbital have a slightly higher energy than s-orbital and so are marginally further from the nucleus so easier to remove
• decrease between group 15 to 16

  • similar decrease to group 2 to 13 ; as you move from group 13 to 18, outer electrons are found in p-orbitals
  • groups 13,14,15 each p-orbitals contain only a single electron
  • group 16, the outermost electron is now spin-paired in the pX orbitals
  • electrons that are spin-paired experience some repulsion - this makes the first outer electron easier to remove ; slightly lower first ionisation energy is observed
• There is a sharp decrease in first ionisation energy :

  • from He at the end of period 1 to Li at the start of period 2
  • from Ne at the end of period 2 to Na at the start of period 3
  • from Ar at the end of period 3 to K at the start of period 4
• trends down a group

  first ionisation energies decrease moving down a group

  • the number of shells increases, so the distance of the outer electrons from the nucleus increases; so, weaker force of attraction on the outer electrons
  • there are more inner shells, so the shielding effect on the outer electrons from the nuclear charge increases; so, weaker force of attraction