Electron configurations and ionisation energies
At A-level, shells split into sub-shells and orbitals. Writing configurations is a skill in itself, and ionisation energies provide the evidence that this model is right.
Part 1 of 3: Learn it
In short
- Sub-shells fill in the order 1s 2s 2p 3s 3p 4s 3d 4p.
- Chromium and copper are exceptions: 3d⁵ 4s¹ and 3d¹⁰ 4s¹.
- First ionisation energy rises across a period and falls down a group, with dips at groups 3 and 6.
Where this is in your specification
Spec points: AQA 7405 3.1.1.3, OCR A H432 2.2.1 and 3.1.1, Edexcel 9CH0 Topic 1
| Board | Topic: Atomic structure and amount of substance |
|---|---|
| AQA 7405 | 3.1.1 and 3.1.2 |
| Edexcel 9CH0 | Topics 1 and 5 |
| OCR H432 | 2.1.1 to 2.1.3, 2.2.1 and 3.1.1 |
| Higher C813 76 | 1(a) Periodicity; 3(a) Getting the most from reactants |
Sub-shells and orbitals
| Sub-shell | Orbitals | Maximum electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
Each orbital holds two electrons with opposite spins. Electrons occupy orbitals of the same sub-shell singly before pairing up.
Writing configurations
- Iron (26 electrons): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s², shortened to [Ar] 3d⁶ 4s².
- Write 3d before 4s in the final answer, but fill 4s first.
- Transition metal ions lose their 4s electrons before 3d: Fe³⁺ is [Ar] 3d⁵.
- Negative ions gain electrons into the next available orbital: Cl⁻ is 1s² 2s² 2p⁶ 3s² 3p⁶.
Ionisation energy
The first ionisation energy is the energy needed to remove one electron from each atom in a mole of gaseous atoms, forming a mole of gaseous 1+ ions: for sodium, Na(g) → Na⁺(g) + e⁻. Three things decide its size: the nuclear charge, the distance of the outer electron from the nucleus, and the shielding by inner electrons.
- Down a group it falls: the outer electron is further away and more shielded.
- Across a period it generally rises: more protons, similar shielding.
- Dip at group 3 (Al below Mg): the outer electron is in a 3p orbital, slightly higher in energy than 3s.
- Dip at group 6 (S below P): one 3p orbital now holds a pair, and the repulsion between paired electrons makes one easier to remove.
Successive ionisation energies
Removing electrons one by one, each takes more energy than the last because the remaining electrons are held by the same nuclear charge with less repulsion. A very large jump shows the next electron is coming from a shell closer to the nucleus, which tells you the group.
Write the full electron configuration of a potassium atom.
Show the answer
1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹.
Part 2 of 3: See it worked
Worked examples
Example 1
An element's successive ionisation energies (kJ mol⁻¹) are 738, 1451, 7733, 10 541. Which group is it in?
- The big jump is between the 2nd and 3rd ionisation energies
- So two electrons are in the outer shell
Answer: Group 2 (it is magnesium).
Example 2
Write the electron configurations of Cu and Cu²⁺.
- Copper has 29 electrons and is an exception: a full 3d with one 4s electron
- Cu: [Ar] 3d¹⁰ 4s¹
- Cu²⁺ loses the 4s electron first, then one 3d electron
Answer: Cu: [Ar] 3d¹⁰ 4s¹; Cu²⁺: [Ar] 3d⁹.
Common mistakes
- Removing 3d electrons before 4s when forming transition metal ions.
- Leaving out (g) in an ionisation equation.
- Explaining the Al dip with pairing, or the S dip with sub-shells. They are the other way round.
- Writing chromium as [Ar] 3d⁴ 4s².
Write the equation for the first ionisation energy of calcium.
Show the answer
Ca(g) → Ca⁺(g) + e⁻.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
Write the full electron configuration of a potassium atom.
1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹.
Write the equation for the first ionisation energy of calcium.
Ca(g) → Ca⁺(g) + e⁻.
Why does first ionisation energy decrease down group 1?
Each step down adds a shell, so the outermost electron sits further out with more inner shells screening it, and the nucleus holds it less tightly.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Diese Lernzettel sind auf Englisch, weil sie britischen Prüfungslehrplänen folgen.
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