Ionisation energy
| English | Chinese | Pinyin |
|---|---|---|
| ionisation energy | 电离能 | diàn lí néng |
| gaseous | 气态 | qì tài |
| nuclear charge | 核电荷 | hé diàn hè |
| atomic radius | 原子半径 | yuán zi bàn jìng |
| shielding | 屏蔽 | píng bì |
| spin-pair repulsion | 自旋成对排斥 | zì xuán chéng duì pái chì |
How tightly are electrons held?
- Ionisation energy 电离能 is the energy to pull electrons off gaseous 气态 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.
The energy to remove one mole of electrons from one mole of gaseous atoms is the first ______ energy.
It measures how tightly the outer electron is held.
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 核电荷 — more protons → stronger pull → higher IE.
- atomic radius 原子半径 — outer electron further out → weaker pull → lower IE.
- shielding 屏蔽 — more inner shells → more shielding → lower IE.
Successive ionisation energies of sodium: big jumps reveal the 2,8,1 shells
Which change would LOWER the first ionisation energy?
More shielding weakens the pull on the outer electron, lowering the ionisation energy.
Match each change to its effect on first ionisation energy.
IE rises with nuclear pull and falls with distance/shielding; successive ionisations always rise.
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 自旋成对排斥).
The ionisation-energy trend — and its dips
First ionisation energy generally rises across a period, but DIPS where a new p sub-shell starts and where a p-orbital pair first forms. Step across to find the two tell-tale dips.
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).
A big jump in successive ionisation energies marks the move to a new, inner shell.
The jumps reveal the shell structure — e.g. sodium's successive IEs split into 1, then 8, then 2.
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)