1.1.5Physical Chemistry
Electron Configuration — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 5 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Rules for Writing Configurations
Three ideas control how electrons are placed.
Lowest energy first
- Lowest energy first Fill orbitals in order of energy: 1s 2s 2p 3s 3p 4s 3d 4p.
Two per orbital
- Two per orbital Each orbital takes a maximum of two electrons with opposite spins.
Singly before pairing
- Singly before pairing Electrons occupy orbitals of equal energy singly first, because pairing causes extra repulsion.
Total
- Total The electron count in the configuration must equal the atomic number for an atom.
Part 2 of 6Examples and Shorthand
Examples and Shorthand
Oxygen
- Oxygen 1s² 2s² 2p⁴, where the 2p electrons are arranged as one pair and two unpaired electrons.
Calcium
- Calcium 1s² 2s² 2p⁶ 3s² 3p⁶ 4s², written in shorthand as [Ar] 4s².
Iron
- Iron 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s², or [Ar] 3d⁶ 4s² in shorthand form.
Shorthand
- Shorthand The previous noble gas in square brackets stands for all its filled inner shells.
Part 3 of 6Chromium and Copper
Half-filled and filled 3d sub-shells give extra stability.
1
Chromium has 24 electrons, and the expected configuration would be [Ar] 3d⁴ 4s².
2
The actual configuration is [Ar] 3d⁵ 4s¹, because one 4s electron moves to give a half-full 3d sub-shell.
3
Copper has 29 electrons, and the expected configuration would be [Ar] 3d⁹ 4s².
4
The actual configuration is [Ar] 3d¹⁰ 4s¹, which gives a full 3d sub-shell.
5
These are the only two exceptions up to krypton that need to be learned.
Part 4 of 6Ions and Blocks
Positive ions
- Positive ions For a d-block atom, 4s electrons are lost before 3d electrons, so Fe²⁺ is [Ar] 3d⁶.
Negative ions
- Negative ions Added electrons go into the next available orbital, so S²⁻ becomes 1s² 2s² 2p⁶ 3s² 3p⁶.
s-block
- s-block Elements whose highest-energy electron is in an s sub-shell, such as Groups 1 and 2.
p- and d-blocks
- p- and d-blocks Elements whose last electron goes into a p or d sub-shell lie in the p-block and d-block respectively.
Part 5 of 6Worked Example
Question Write the full electron configurations of an iron atom, an Fe³⁺ ion and a sulfide ion S²⁻.
- Iron has 26 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s².
- Fe³⁺ has 23 electrons: remove the two 4s electrons first, then one 3d electron.
- So Fe³⁺ is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵.
- Sulfur has 16 electrons, and S²⁻ has 18: 1s² 2s² 2p⁶ 3s² 3p⁶.
Answer
- Fe: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²;
- Fe³⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵;
- S²⁻: 1s² 2s² 2p⁶ 3s² 3p⁶
Part 6 of 6Exam Checklist & Tips
1.1.5 – Key Points for the Exam
- Electron configuration The arrangement of electrons in the shells, sub-shells and orbitals of an atom or ion.
- s-block The elements whose outermost electron is in an s sub-shell.
- d-block The elements whose highest-energy electron is in a d sub-shell.
- Noble gas core The filled shells of the previous noble gas, written in square brackets as shorthand.
Common Mistakes
- Removing 3d electrons before 4s electrons when forming a transition metal ion.
- Writing chromium as 3d⁴ 4s² and copper as 3d⁹ 4s² instead of the correct exceptions.
- Pairing electrons in a p sub-shell before each orbital has one electron.
- Forgetting to add the ion charge to the electron count when writing the configuration of an ion.
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