1.2.4Physical Chemistry
Electronic Configurations — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 8 of 42
Section 1 of 5Building the Configuration
The order of filling follows the energy of the subshells, not simply their numbering.
1
Find the number of electrons: equal to the atomic number for atoms, adjusted for any ionic charge.
2
Fill subshells in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.
3
Do not exceed the subshell capacities of 2 (s), 6 (p) and 10 (d).
4
Write the result as numbers for shells and superscripts for electron counts, such as 1s² 2s² 2p⁶.
5
A shorthand uses the previous noble gas in brackets, so potassium is [Ar] 4s¹.
Section 2 of 5Examples
Carbon
- 1s² 2s² 2p², where the two 2p electrons occupy separate orbitals.
Calcium
- 1s² 2s² 2p⁶ 3s² 3p⁶ 4s², showing that 4s fills before 3d.
Iron
- [Ar] 3d⁶ 4s², with the 3d subshell filling after the 4s.
Chloride ion
- 1s² 2s² 2p⁶ 3s² 3p⁶ for Cl⁻, which has the same configuration as argon.
| Species | Electrons | Configuration |
|---|---|---|
| O²⁻ | 10 | 1s² 2s² 2p⁶ |
| Na⁺ | 10 | 1s² 2s² 2p⁶ |
| Ca | 20 | [Ar] 4s² |
| Zn | 30 | [Ar] 3d¹⁰ 4s² |
Section 3 of 5Exceptions and Ions
Exceptions and Ions
Chromium
- [Ar] 3d⁵ 4s¹ rather than 3d⁴ 4s², because a half-filled 3d subshell is more stable.
Copper
- [Ar] 3d¹⁰ 4s¹ rather than 3d⁹ 4s², because a completely filled 3d subshell is more stable.
Positive ions
- Electrons are removed from the highest energy subshell, which for transition elements means 4s before 3d.
Negative ions
- Extra electrons are added to the next available orbital in the outer subshell.
Section 4 of 5Worked Example
Question Write full electronic configurations for sulfur (Z = 16), the sulfide ion S²⁻, and iron (Z = 26) in noble-gas shorthand.
- Sulfur has 16 electrons: 1s² 2s² 2p⁶ 3s² 3p⁴ (2 + 2 + 6 + 2 + 4 = 16).
- S²⁻ has gained two electrons, so 18 in total: 1s² 2s² 2p⁶ 3s² 3p⁶, the same as argon.
- Iron has 26 electrons, which is argon's 18 plus 8 more.
- The extra 8 fill 4s first (2) then 3d (6), giving [Ar] 3d⁶ 4s².
Answer
- S: 1s² 2s² 2p⁶ 3s² 3p⁴;
- S²⁻: 1s² 2s² 2p⁶ 3s² 3p⁶;
- Fe: [Ar] 3d⁶ 4s².
Section 5 of 5Exam Checklist & Tips
1.2.4 – Key Points for the Exam
- Electronic configuration The arrangement of electrons in shells and subshells of an atom or ion.
- Aufbau principle Electrons fill the lowest available energy subshell first.
- Noble gas core Shorthand in square brackets representing the filled inner electrons of the previous noble gas.
- Isoelectronic Having the same electronic configuration, such as O²⁻ and Na⁺.
Common Mistakes
- Filling 3d before 4s for neutral atoms.
- Removing 3d electrons before 4s electrons when forming a transition metal cation.
- Giving chromium and copper the regular patterns instead of their half-filled and fully filled subshells.
- Forgetting to adjust the electron count for ions.
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