Ionisation Energies (Atomic Structure)

Define ionisation energy

The energy needed to remove one mole of electrons from one mole of atoms in a gaseous state

What are the three factors that affect ionisation energy?

1. The distance of the electron being removed from the nucleus

2. The number of protons in the nucleus

3. The amount of shielding

State the trend of the 1st ionisation energy down a group. Give an explanation.

• 1st ionisation energy decreases down a group

• As you go down:

  • Atomic radius increase

  • There is more shielding

  So the attraction between the nucleus and e- to be removed decreases, so less energy is required

State the trend of the 1st ionisation energy across a period. Give an explanation.

• General increase

• As you go across the period

  • e- to be removed is in the same shell

  • so similar shielding

  • There are more protons in nucleus

  • so attraction between nucleus and outer electron increases

    so more energy is required to remove outer electron

Why is there a small dip in ionisation energy between group 2 and group 3 elements?

• Electron being removed in group 3 is in p subshell

• Electron being removed in group 2 is in s subshell

• p subshell is further away from the nucleus and more shielded

• so there is less attraction between the nucleus and outer electron in group 3

Why is there a small dip in ionisation energy between group 5 and group 6 elements?

• In group 6 there is a pair of electrons in the p orbital

• These repel and so the electrons are easier to remove

Why do successive ionisation energies increase?

Each subsequent electron needs more energy as it is being removed from a more positive ion, so the attraction is greater