Electron configurationAQA A-Level Chemistry: Revision notes
Section 1
Shells, sub-shells and orbitals
Electrons are arranged in shells (principal quantum numbers 1, 2, 3, 4), which contain sub-shells labelled s, p and d. Each sub-shell is made up of orbitals, and an orbital can hold a maximum of two electrons, which must have opposite spins.
| Sub-shell | Orbitals | Maximum electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
Shell 1 has only an s sub-shell; shell 2 has s and p; shell 3 and above have s, p and d.
Section 2
Filling order and the electron configuration of atoms
Electrons occupy the lowest available energy level first. Up to Z = 36 the order is
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p
The 4s sub-shell is lower in energy than 3d, so it fills first. In the electron configuration, the number of electrons is a superscript. Examples:
- Cl (Z = 17): 1s² 2s² 2p⁶ 3s² 3p⁵
- Fe (Z = 26): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²
- Br (Z = 35): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁵
When electrons are placed in orbitals of the same sub-shell, they occupy them singly with parallel spins before pairing up. Nitrogen's 2p³ electrons occupy three separate p orbitals.
The block of an element is the type of sub-shell that holds its highest-energy electron: s-block, p-block or d-block.
Section 3
Two exceptions: chromium and copper
Chromium and copper do not follow the simple order, because one 4s electron moves into the 3d sub-shell to give a more stable half-full or full 3d sub-shell.
- Cr (Z = 24): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹ (not 3d⁴ 4s²)
- Cu (Z = 29): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹ (not 3d⁹ 4s²)
Writing Cr as 3d⁴ 4s² or Cu as 3d⁹ 4s² loses the mark. Learn both exceptions.
Section 4
Electron configurations of ions
For positive ions, remove electrons from the outermost shell first. For d-block elements the 4s electrons are lost before the 3d electrons, even though 4s fills first.
- Fe²⁺: remove two 4s electrons from 3d⁶ 4s² → 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶
- Fe³⁺: then one 3d electron → 3d⁵
- Cu²⁺: lose 4s¹ and one 3d → 3d⁹
For negative ions, add electrons to the next available place, e.g. Cl⁻ is 1s² 2s² 2p⁶ 3s² 3p⁶ and Se²⁻ is ...4s² 4p⁶.
Do not remove 3d electrons first when making a transition metal ion. The number of electrons in the ion is Z minus the charge.
Section 5
Worked example
Write the electron configuration of Ni²⁺ (Z = 28).
- Nickel atom: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸ 4s²
- Remove the two 4s electrons.
- Ni²⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸ (26 electrons = 28 − 2)
Check that the total number of electrons equals Z minus the charge.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Electron configuration
- Transition metal ions give many gemstones their colour. Ruby owes its red colour to Cr³⁺ ions in aluminium oxide, and a pale green mineral contains Fe²⁺ ions. The atomic numbers of chromium and iron are 24 and 26.Write the full electron configuration of a Fe²⁺ ion, and state which sub-shell the electrons were removed from when the ion formed.2 marks
- Semiconductor devices are made from p-block elements such as gallium (atomic number 31) and selenium (atomic number 34). A technician needs to know their electron configurations and those of their ions.Write the full electron configuration of a gallium atom and explain why gallium is classed as a p-block element.2 marks
- A student writes the ground-state electron configuration of titanium (atomic number 22) as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴, and states that when titanium forms ions the electrons are lost from the 3d sub-shell first. Vanadium has atomic number 23.Identify the two errors in the student's statements, and write the correct ground-state electron configuration of titanium.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).