9701 · 1.4
Ionisation energy
Ionisation energy is the energy needed to pull an electron away from an atom. This 'cost' changes predictably, telling us a lot about an atom's structure.
Need to know
What you need to know
- Energy is always required, so ionisation is an endothermic process (ΔH is positive).
- Measured in kilojoules per mole (kJ mol⁻¹).
- The lower the ionisation energy, the more easily an atom forms a positive ion.
Explanation
The Cost of Removing an Electron
- First IE is the energy needed to remove 1 mole of the outermost electrons from 1 mole of gaseous atoms, forming 1+ ions.
- Across a period, IE generally increases. The nuclear charge gets stronger, pulling the outer shell in closer, making electrons harder to remove.
- Down a group, IE decreases. The atom gets bigger with more shells, so the outer electron is further away and better shielded from the nucleus.
- Small dips occur from Group 2 to 3 (a new, higher-energy p-subshell) and Group 5 to 6 (repulsion from paired electrons in a p-orbital).