A-level Chemistry · AQA + OCR A
Block Count: electron configurations from the periodic table
Use the periodic table on your data sheet to work out the subshell and its occupancy without being told the period, group or atomic number.
Make the periodic table do some of the work
Keep the periodic table on your data sheet open. Each round gives you an element and nothing else. Find the block, count across it, and use the row to work out the subshell.
Shells, subshells and ions
Shell, subshell or orbital?
The number in 3p⁴ identifies principal shell 3. The letter identifies a p subshell. The superscript says that subshell contains four electrons.
An s subshell contains one orbital, a p subshell three and a d subshell five. Each orbital holds at most two electrons with opposite spins.
Check the ending from the periodic table
Use the table as a quick check. For O, count four places across the Period 2 p block, so the final p occupancy is 2p⁴. For Cl, count five across the Period 3 p block, giving 3p⁵.
In the d block, use one shell number lower: Fe is sixth across the Period 4 d block, so the d occupancy is 3d⁶. Cr and Cu are the two exceptions in this range. Practise this with Block Count.
Capacity and filling order
The maximum capacities of shells 1–4 are 2, 8, 18 and 32 electrons. For the neutral atoms shown here, use 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p, with the chromium and copper exceptions where they are in your course.
The 4s subshell therefore starts to fill while shell 3 still has spare capacity.
What happens to 4s when an ion forms?
For the positive d-block ions used here, remove 4s electrons before 3d electrons. For example, Fe is [Ar] 3d⁶ 4s² and Fe²⁺ is [Ar] 3d⁶.
The relative energies of 4s and 3d depend on the electronic configuration. Use the filling sequence to build the neutral atom, then use the removal rule above to form its ion.
Why are chromium and copper different?
The observed ground states are Cr: [Ar] 3d⁵ 4s¹ and Cu: [Ar] 3d¹⁰ 4s¹. I'd learn the Cr and Cu occupancies your course expects. Their configurations reflect the detailed relative energies of the 3d and 4s electrons in these many-electron atoms.
OCR Year 12 / AS practice here leaves these exceptions and d-block ions for the full A-level extension. AQA atom and ion practice includes them.
How should I type the configuration?
Type 1s2 2s2 2p6, 1s² 2s² 2p⁶ or 1s^2 2s^2 2p^6. We accept 3d before 4s or 4s before 3d. Spaces are optional; the occupancies need to be correct.
When the question asks for a full configuration, expand [He], [Ne] or [Ar]. Formatting fixes leave your attempt count unchanged.
Use configurations with ionisation energy
A large jump in successive ionisation energies indicates removal from an inner shell. Smaller changes can involve different subshell energies or repulsion between paired electrons.
Practise ionisation-energy reasoning or apply configurations to periodicity. Full A-level students can also use the transition-metal questions.
OCR A assesses orbital-box diagrams and orbital shapes; AQA students can use them here as optional support. Unpaired-electron counting appears in both boards. Ligand-field electron arrangements are outside this resource.