1.2.2Physical Chemistry
Quantum Shells — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 6 of 42
Part 1 of 5Shells and the Principal Quantum Number
The Bohr-style shell model is a simple but powerful description of where electrons are found.
Quantum shells
- Quantum shells Electrons occupy fixed energy levels called shells, numbered by the principal quantum number n = 1, 2, 3, 4 outwards from the nucleus.
Energy
- Energy Shells further from the nucleus have higher energy, and electrons fill the lowest available energy first.
Capacity
- Capacity Shell n can hold up to 2n² electrons, giving 2, 8, 18 and 32 electrons for n = 1 to 4.
Evidence
- Evidence Large jumps in successive ionisation energies show that electrons are grouped into shells.
Part 2 of 5Subshells
Subshells
Types
- Types Each shell is divided into subshells named s, p, d and f, which differ slightly in energy.
Number per shell
- Number per shell Shell n contains n subshells, so n = 1 has only 1s, n = 2 has 2s and 2p, n = 3 has 3s, 3p and 3d.
Capacity
- Capacity The s, p, d and f subshells hold a maximum of 2, 6, 10 and 14 electrons respectively.
Order
- Order Within a shell the energy order is s < p < d < f, but the 4s subshell fills before 3d.
| Shell (n) | Subshells | Max electrons |
|---|---|---|
| 1 | 1s | 2 |
| 2 | 2s 2p | 8 |
| 3 | 3s 3p 3d | 18 |
| 4 | 4s 4p 4d 4f | 32 |
Part 3 of 5Using Ionisation Data
1
First ionisation energies across a period increase with nuclear charge but show small dips that point to subshell structure.
2
The drop from Be to B shows that the 2p electron is higher in energy than the 2s electrons.
3
The drop from N to O shows pairing of electrons in a p orbital, which adds repulsion.
4
Successive ionisation energies show shells as large jumps, while the smaller dips reveal subshells.
Part 4 of 5Worked Example
Question
- How many subshells and how many electrons can the third shell hold?
- Give the subshells in order of capacity.
- Shell n = 3 contains 3 subshells: 3s, 3p and 3d.
- Capacities are 2, 6 and 10 electrons respectively.
- Total = 2 + 6 + 10 = 18, which agrees with 2n² = 2 × 3² = 18.
Answer Three subshells (3s, 3p, 3d) holding up to 18 electrons.
Part 5 of 5Exam Checklist & Tips
1.2.2 – Key Points for the Exam
- Principal quantum number The number n that identifies a shell and its relative energy.
- Subshell A division of a shell, labelled s, p, d or f.
- Shell capacity The maximum number of electrons in shell n, equal to 2n².
- Energy level A fixed amount of energy that an electron in an atom can have.
Common Mistakes
- Saying electrons orbit the nucleus in fixed circular paths; they occupy regions of space with fixed energies.
- Stating that shell 3 holds only 8 electrons in all cases; its full capacity is 18.
- Forgetting that 4s is filled before 3d.
- Mixing up the number of subshells with the number of electrons in a shell.
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