Pearson Edexcel Edexcel 2021-style practice: (a) Write the full electron configuration of an iron atom and of an Fe^2+ ion. (b) Explain why the first ionisation energy of sulfur is lower than that of phosphorus.

Use the 4s-before-3d fill but 4s-lost-first rule, then the pairing argument. (a) Fe: 1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 4s^2 (1). Fe^2+: 1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 (4s electrons lost first) (1). (b) In phosphorus the 3p electrons are unpaired (half-filled 3p^3); in sulfur the fourth 3p electron is paired in an orbital (1), and the electron-electron repulsion between the paired electrons makes it easier to remove, lowering the first ionisation energy (1).

EnglandChemistrySyllabus dot point

How does the arrangement of sub-atomic particles and electrons explain the structure of the periodic table?

Sub-atomic particles, isotopes and mass spectrometry, electronic configuration in sub-shells, ionisation energies and the evidence they provide for shell and sub-shell structure.

An Edexcel 9CH0 Topic 1 answer covering protons, neutrons and electrons, isotopes and mass spectrometry, electron configuration in sub-shells, and the ionisation energy evidence for shell structure.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this topic is asking
Sub-atomic particles and isotopes
Mass spectrometry
Electron configuration
Ionisation energy as evidence
Examples in context
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What this topic is asking

Edexcel Topic 1 wants you to describe the atom in terms of protons, neutrons and electrons, work with isotopes and mass spectra, write electron configurations using s, p and d sub-shells, and use successive and first ionisation energy data as evidence for the shell and sub-shell model.

Sub-atomic particles and isotopes

Protons and neutrons sit in the nucleus; electrons occupy shells around it. The proton (relative mass 1, charge $+1$ ) and neutron (relative mass 1, charge $0$ ) dominate the mass; the electron (relative mass $\frac{1}{1836}$ , charge $-1$ ) is negligible by mass.

Mass spectrometry

A time-of-flight mass spectrometer ionises a sample, accelerates the ions through an electric field, and separates them by mass-to-charge ratio ( $m/z$ ). The relative atomic mass is the weighted mean of the isotope masses:

A_r = \frac{\sum (\text{isotope mass} \times \text{abundance})}{\sum \text{abundance}}

Electron configuration

Electrons fill sub-shells in order of increasing energy: $1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p$ . Note that $4s$ fills before $3d$ but empties first on ionisation. For example, iron is $1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 4s^2$ .

Ionisation energy as evidence

Successive ionisation energies for one atom rise steadily, then jump sharply when an electron is removed from a shell closer to the nucleus. These large jumps reveal the number of electrons in each shell. Small dips within a period (for example, the drop from Group 2 to Group 3, and Group 5 to Group 6) provide evidence for sub-shells and electron pairing.

Using successive ionisation energies to find the group

The first five successive ionisation energies of an element (in $\text{kJ mol}^{-1}$ ) are $738$ , $1451$ , $7733$ , $10540$ , $13630$ . Deduce which group of the periodic table the element is in.

step 1: Look for the large jump

The values rise gently from the 1st to the 2nd, then jump sharply (more than fivefold) between the 2nd and 3rd ionisation energies.

step 2: Interpret the jump

A large jump means the next electron is removed from a shell closer to the nucleus (a new inner shell), which is much harder to remove.

step 3: Count the easily removed electrons

Two electrons are removed relatively easily before the jump, so there are two electrons in the outer shell.

step 4: Conclude

Two outer-shell electrons place the element in Group 2 (this data is for magnesium). The pattern of jumps is direct evidence for the shell model of the atom.

Examples in context

Example 1. Dating with mass spectrometry. Mass spectrometers measure the relative abundances of isotopes, which is how carbon dating works: the ratio of $^{14}\text{C}$ to $^{12}\text{C}$ in a once-living sample falls predictably as the radioactive $^{14}\text{C}$ decays. The same time-of-flight instrument used to find relative atomic masses in the laboratory underpins archaeological dating, showing that isotope abundance is a measurable, useful quantity.

Example 2. Flame tests and electron transitions. When metal ions are heated in a flame, electrons are promoted to higher shells and then fall back, emitting light of characteristic colour (sodium yellow, potassium lilac). Although flame colours involve emission rather than ionisation, they are direct evidence that electrons occupy discrete energy levels, the same shell structure that successive ionisation energies reveal. This links the abstract sub-shell model to a vivid, testable observation.

Try this

Q1. State what is meant by the first ionisation energy. [2 marks]

Cue. Energy to remove one electron from each atom in one mole of gaseous atoms.

Q2. Explain why the first ionisation energy of magnesium is higher than that of aluminium. [2 marks]

Cue. Aluminium's outer electron is in a $3p$ sub-shell, which is higher in energy and more shielded than magnesium's $3s$ electron, so it is easier to remove.

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 20184 marksChlorine has two isotopes,

^{35}\text{Cl}

and

^{37}\text{Cl}

, with relative abundances of

75.8\%

and

24.2\%

. (a) Calculate the relative atomic mass of chlorine. (b) State why the two isotopes have identical chemical properties.

Show worked answer →

Take the weighted mean of the isotope masses, then explain in terms of electrons.

(a) $A_r = \dfrac{(35 \times 75.8) + (37 \times 24.2)}{100}$ (1) $= \dfrac{2653 + 895.4}{100} = \dfrac{3548.4}{100} = 35.5$ (to 3 s.f.) (2).

(b) The isotopes have the same number of electrons and the same electron configuration, and chemical properties depend on the electrons, not the number of neutrons (1).

Edexcel 20214 marks(a) Write the full electron configuration of an iron atom and of an

\text{Fe}^{2+}

ion. (b) Explain why the first ionisation energy of sulfur is lower than that of phosphorus.

Show worked answer →

Use the $4s$ -before- $3d$ fill but $4s$ -lost-first rule, then the pairing argument.

(a) Fe: $1\text{s}^2 2\text{s}^2 2\text{p}^6 3\text{s}^2 3\text{p}^6 3\text{d}^6 4\text{s}^2$ (1). $\text{Fe}^{2+}$ : $1\text{s}^2 2\text{s}^2 2\text{p}^6 3\text{s}^2 3\text{p}^6 3\text{d}^6$ ( $4\text{s}$ electrons lost first) (1).

(b) In phosphorus the $3\text{p}$ electrons are unpaired (half-filled $3\text{p}^3$ ); in sulfur the fourth $3\text{p}$ electron is paired in an orbital (1), and the electron-electron repulsion between the paired electrons makes it easier to remove, lowering the first ionisation energy (1).

Related dot points

Sources & how we know this

Pearson Edexcel A-Level Chemistry (9CH0) specification — Pearson Edexcel (2015)