Ionisation energy
Ionisation energy
- Ionisation energy is the energy to pull electrons off gaseous atoms.
- Its trends and dips are powerful evidence for shells and sub-shells.
- It is a favourite exam topic.
First and successive ionisation energies
- The first ionisation energy removes one electron from each atom in a mole of gaseous atoms (unit $\text{kJ}/\text{mol}$):
$$\text{X}(\text{g}) \rightarrow \text{X}^{+}(\text{g}) + \text{e}^{-}$$
- Successive ionisation energies remove the next electron each time, e.g. $\text{X}^{+}(\text{g}) \rightarrow \text{X}^{2+}(\text{g}) + \text{e}^{-}$.
- Each successive value is larger — the electron leaves a more positive ion.
Why are gaseous atoms specified in the definition of ionisation energy?
Using gaseous atoms means there are no interactions between particles, so the value measures only the atom's own attraction.
Each successive ionisation energy is larger than the last because:
After each removal the ion is more positive, so the next electron is held more tightly.
What it depends on
- nuclear charge — more protons → stronger pull → higher IE.
- atomic radius — outer electron further out → weaker pull → lower IE.
- shielding — more inner shells → more shielding → lower IE.
Which change would LOWER the first ionisation energy?
More shielding weakens the pull on the outer electron, lowering the ionisation energy.
Trends and the tell-tale dips
- Across a period IE generally rises; down a group it falls.
- But two dips appear in Period 3 — evidence for sub-shells:
- Al dip: its electron comes from a higher-energy 3p sub-shell.
- S dip: a paired 3p electron is easier to remove (spin-pair repulsion).
The dip in first ionisation energy at aluminium (vs magnesium) is because:
Al's outer electron is in 3p (higher energy than Mg's full 3s), so it is easier to remove — a dip.
Successive IE → shells
- Big jumps appear when the next electron must come from a shell closer in.
- Counting the electrons removed before the first big jump gives the element's group.
A big jump in successive ionisation energies happens when:
The big jump marks moving to an inner shell; counting electrons before the first jump gives the outer-shell number (the group).
You've got it
- 1st IE: $\text{X}(\text{g}) \rightarrow \text{X}^{+}(\text{g}) + \text{e}^{-}$ (gaseous, $\text{kJ}/\text{mol}$); successive values keep rising
- IE depends on nuclear charge, radius, shielding
- across a period IE rises (dips at Al = 3p starts, S = spin-pair); down a group it falls
- big jumps in successive IE reveal the shell structure (and the group)