イオン化エネルギー

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.

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

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).

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.

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