QCE Chemistry - Unit 1 - Properties and structure of atoms

Electron configurations and ionisation-energy evidence

Learn electron configurations for QCE Chemistry Unit 1 with worked reasoning, KaTeX equations, original diagrams and assessment checks.

Part of the free QCE Chemistry notes library for Unit 1: Properties and structure of atoms.

Updated 2026-08-10 - 7 min read

Write full and condensed configurations to Z = 36 and infer occupied energy levels from successive ionisation energies. This note is designed to be used actively: pause at each prediction, show the particle-level or quantitative reason, and only then compare your reasoning with the worked answer. The aim is not to collect definitions. It is to build a chemical model that remains dependable when the substances, data or experimental context change.

The central chemical model

Electrons occupy available orbitals from lower to higher energy. Aufbau sets the filling order, Pauli limits an orbital to two opposite-spin electrons, and Hund places electrons singly across equal-energy orbitals before pairing.

Move between the three levels

• Observable level: Successive ionisation energies rise gradually, then may show a very large jump after the valence electrons have been removed.
• Particle level: Electrons occupy orbitals rather than circular tracks; core electrons are closer to the nucleus and experience stronger effective attraction than valence electrons.
• Symbolic level: Fe is [Ar]4s²3d⁶, while Fe²⁺ is [Ar]3d⁶ because the n = 4 electrons are removed first.

Configuration notation records electron allocation; an orbital-box diagram audits Hund and Pauli; ionisation data supplies experimental evidence about shell occupancy.

[!MODEL BOUNDARY]

The Aufbau sequence is a useful ground-state ordering for the required atoms to Z = 36, not a complete quantum-mechanical energy calculation or a picture of electron paths.

The exact relationship

$ 1s<2s<2p<3s<3p<4s<3d<4p $

This is the filling sequence used for the required atoms and ions up to Z = 36.

Before substituting values, name what each symbol or chemical formula represents in this context. Check units, state symbols and signs. After calculating, test whether the magnitude and direction are chemically plausible. A calculator can execute arithmetic but cannot tell you that an isotope average lies outside the isotope range, a negative absolute temperature was used, or an ionic formula carries a residual charge.

A repeatable reasoning method

1. Count electrons, adjusting for ionic charge before filling.
2. Fill in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.
3. Use a noble-gas core for condensed notation.
4. For transition-metal cations, remove 4s electrons before 3d electrons; retain the Cr and Cu atom exceptions.

This sequence is a reasoning scaffold, not a sentence template. In a short-response question, compress it to the decisions that earn marks. In a practical or data question, keep the evidence visible: name the observation, quote or process relevant data, and explain how the model supports the conclusion. If the question asks you to analyse, do more than state a trend—use the trend to infer a structure, process or relationship.

Evidence clinic: Read a successive-ionisation fingerprint

Scenario. An element has successive ionisation energies 738, 1451, 7733 and 10 542 kJ mol⁻¹. Infer its valence-electron count and likely group.

| Observed or given | Chemical meaning | | --- | --- | | IE₁ to IE₂ increases moderately | The first two electrons are removed from the same outer shell. | | IE₂ to IE₃ increases by more than fivefold | After two removals, the next electron is a core electron in a lower shell. | | Two electrons are removed before the jump | The neutral atom has two valence electrons and is consistent with Group 2. |

Analysis. The absolute values matter less than the discontinuity. The first large jump locates the boundary between valence and core electrons; it does not mean the atom changes element during ionisation.

Defensible conclusion. The element has two valence electrons and is consistent with a Group 2 configuration such as [noble gas]ns².

[!LIMIT OF THE EVIDENCE]

The jump pattern identifies valence-shell structure more securely than a unique element; period or identity needs additional evidence.

Worked example

The final answer is only the last line of the reasoning. To learn from the example, cover the steps and reproduce them from the prompt. Then change one feature—an ionic charge, quantity, temperature, molecular shape or measured interval—and predict which steps must change and which chemical principle stays invariant.

Why this matters in unfamiliar questions

A large jump in successive ionisation energy appears after all valence electrons have been removed. The jump is evidence that the next electron belongs to a lower, more strongly attracted shell.

QCAA-style questions often provide enough information but distribute it across prose, a diagram and a data table. Start by translating every given item into a chemical role. Mark values that are initial, final, measured or derived. Identify controlled variables before comparing trials. If a conclusion depends on more than one observation, state how the observations work together. Avoid claiming certainty beyond the resolution of the method.

For quantitative work, write the governing relationship before numbers, preserve unrounded intermediate values and round only the final answer to a precision justified by the data. For explanatory work, use a cause chain: structural or experimental change → particle-level consequence → change in collisions, attractions, energy or composition → observed result. That chain is more transferable than a memorised trend.

How to judge practical or data evidence

Use the clinic above as a model: quote the relevant observation, translate it into particle or quantitative meaning, and then state a conclusion no stronger than the method allows. A valid comparison changes one independent variable, defines the dependent measure and controls plausible alternative causes. Replicates reveal random variation; they do not repair a calibration bias, heat loss, contamination or an unsuitable measurement range.

For laboratory work, name hazards that actually arise from the substances and procedure. Reduce risk through concentration, scale, containment, ventilation, temperature control and disposal design before relying on personal protective equipment alone.

Common mistake and repair

The repair is important because Chemistry marking rewards the relationship that justifies an answer. Before finishing, audit four things: particle identity, conserved atoms or charge, direction of energy or matter transfer, and units. If all four remain consistent, the explanation is usually much harder to derail.

Try it yourself

Now answer these without returning to the note:

• How are equal-energy p orbitals filled initially?
• Which atom has the exceptional configuration [Ar]4s¹3d⁵?
• Which electrons does Fe lose first?
• What does a large successive-ionisation jump reveal?

For each response, add a brief verification: charge balance, atom count, a reverse substitution, a limiting case, a particle sketch or a check against the graph. Verification turns a plausible answer into a defensible one.

Assessment transfer checklist

• I can define the relevant model without circular wording.
• I can represent it with the required formula, equation, state symbols or diagram.
• I can show why the observation follows from particles, forces, collisions, energy or amount.
• I can calculate with units and retain sensible precision.
• I can distinguish direct evidence from an inference.
• I can state a limitation without claiming that all evidence is therefore useless.

Sources

• QCAA Chemistry subject page
• QCAA Chemistry 2025 syllabus
• OpenStax Chemistry 2e
• IUPAC Gold Book

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