Topic 1: Atomic Structure

CIE Chemistry (9701) › Topic 1

AS Level · Physical Chemistry · CIE 9701 · 2025–2027 syllabus

Topic 1: Atomic Structure

AS Level · Physical Chemistry · Topic 1 of 22
Atomic Structure

The foundation of all chemistry — understanding the subatomic particles, electron arrangement, and nuclear notation that underpin every topic that follows. Tested in Paper 1 (MCQ) and Paper 2 (Structured). Key skills: writing electron configurations, interpreting mass spectra, explaining ionisation energy trends.

3Subatomic particles
P1+P2Papers assessed
~8%Typical P1 share
HighExam frequency

What this topic covers

Subatomic particles
Protons, neutrons, electrons — their relative masses, charges and locations in the atom.
Isotopes
Atoms of the same element with different numbers of neutrons. Same Z, different A.
Mass spectrometry
How a mass spectrometer works; interpreting spectra to find relative atomic mass (Ar).
Electron configuration
Writing full and abbreviated configs using subshell notation (1s² 2s² 2p⁶…) and explaining exceptions (Cr, Cu).
Ionisation energy
First and successive ionisation energies. Trends across periods and down groups. Evidence for shell structure.
Atomic orbitals
Shapes of s, p and d orbitals. Aufbau principle, Hund's rule and the Pauli exclusion principle.
Why this topic matters: Electron configuration reappears in bonding (Topic 3), periodicity (Topic 9), transition metals (Topic 28), and throughout organic chemistry (bond polarity, lone pairs). Ionisation energy data questions appear almost every year in Paper 2.

Key particles at a glance

Particle Symbol Relative Mass Relative Charge Location
Proton p⁺ 1 +1 Nucleus
Neutron n⁰ 1 0 Nucleus
Electron e⁻ 1/1840 (≈ 0) −1 Shells / orbitals
Common slip: The mass of an electron is often stated as zero in calculations (it is negligible), but never write that its charge is zero — it is −1 and defines the element's chemistry.

Full revision notes · aligned with 9701 syllabus learning outcomes

Revision Notes

A. Nuclear Notation & Isotopes

Every atom is represented as ᴬZX, where A = mass number (protons + neutrons) and Z = atomic number (protons). The number of neutrons = A − Z.

Isotopes are atoms of the same element (same Z) with different numbers of neutrons (different A). They have identical chemical properties but different physical properties (e.g. melting point, rate of diffusion). Example: ¹²C and ¹³C and ¹⁴C are all isotopes of carbon (Z = 6).

B. Mass Spectrometry

A mass spectrometer separates ions by their mass-to-charge (m/z) ratio. The key steps are:

  1. 1. VaporiseSample is vaporised into gaseous atoms/molecules.
  2. 2. IoniseHigh-energy electrons knock out one electron from each atom → positive ions (M⁺).
  3. 3. AccelerateElectric field accelerates ions to the same kinetic energy.
  4. 4. DeflectMagnetic field deflects ions — lighter ions deflect more, heavier ions deflect less.
  5. 5. DetectDetector records the m/z and relative abundance of each ion.
Calculating Aᵣ from a mass spectrum: Multiply each isotope's mass by its % abundance, sum the results, divide by 100. e.g. for chlorine (75% ³⁵Cl, 25% ³⁷Cl): Aᵣ = (75 × 35 + 25 × 37) / 100 = 35.5

C. Electron Configuration

Electrons occupy subshells in order of increasing energy. The four rules to follow:

Aufbau principle
Electrons fill the lowest energy subshell first. Order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p…
Pauli exclusion principle
Each orbital holds a maximum of 2 electrons with opposite spins (↑↓).
Hund's rule
Electrons occupy orbitals singly before pairing. All single electrons have parallel spins.
Exceptions: Cr & Cu
Cr is [Ar] 3d⁵ 4s¹; Cu is [Ar] 3d¹⁰ 4s¹ — half-filled/full d subshells are extra stable.

Common electron configurations (Period 1–3 + key exceptions)

H
Z = 1
1s¹
He
Z = 2
1s²
C
Z = 6
1s² 2s² 2p²
Ne
Z = 10
1s² 2s² 2p⁶
Na
Z = 11
1s² 2s² 2p⁶ 3s¹
Cl
Z = 17
1s² 2s² 2p⁶ 3s² 3p⁵
Cr
Z = 24
[Ar] 3d⁵ 4s¹ ⚠
Cu
Z = 29
[Ar] 3d¹⁰ 4s¹ ⚠

D. Ionisation Energy

The first ionisation energy (1st IE) is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions:

X(g) → X⁺(g) + e⁻    ΔH = 1st IE (always positive / endothermic)

Successive ionisation energies increase because each subsequent electron is removed from an increasingly positive ion. A large jump between successive IEs indicates that the next electron is being removed from a new (inner) shell — this is how you can deduce the group of an element.

Trend across a period →
Generally increases. More protons, same shielding, stronger nuclear attraction. Dips at group 3 (new subshell) and group 6 (paired electron repulsion).
Trend down a group ↓
Decreases. Electrons are further from nucleus with more shielding from inner shells, so the outer electron is less strongly attracted.

Fully worked solutions with examiner mark scheme annotations

Worked Examples

Example 1 · Mass spectrum calculation · Paper 1 / Paper 2 style
Boron has two naturally occurring isotopes: ¹⁰B (19.9%) and ¹¹B (80.1%). Calculate the relative atomic mass of boron to 1 decimal place.
  1. Multiply each isotope's mass by its percentage abundance:
    ¹⁰B: 10 × 19.9 = 199  ·  ¹¹B: 11 × 80.1 = 881.1
  2. Sum the results: 199 + 881.1 = 1080.1
  3. Divide by 100: 1080.1 ÷ 100 = 10.801
  4. Round to 1 d.p.: Aᵣ = 10.8
✓ Answer: Aᵣ(B) = 10.8
Example 2 · Electron configuration · AS Paper 1 style
Write the full electron configuration of a Cu²⁺ ion.
  1. Write the ground state configuration of Cu (note the exception): Cu = [Ar] 3d¹⁰ 4s¹
  2. Cu²⁺ has lost 2 electrons. Remove from the highest energy subshell first — 4s before 3d.
  3. Remove 1 electron from 4s¹ → [Ar] 3d¹⁰ (Cu⁺)
  4. Remove 1 more electron from 3d¹⁰ → [Ar] 3d⁹ (Cu²⁺)
✓ Answer: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁹
Example 3 · Successive ionisation energies · AS Paper 2 style
The successive ionisation energies (kJ mol⁻¹) for an element X are: 1090, 2350, 4610, 6220, 37 800, 47 300. Identify which group X belongs to and explain your reasoning.
  1. Look for the large jump in successive IEs: the jump occurs between the 4th IE (6220) and 5th IE (37 800).
  2. The large jump means the 5th electron is being removed from an inner shell (much closer to the nucleus).
  3. Therefore, there are 4 electrons in the outer shell → X is in Group 14.
✓ Answer: Group 14. The large jump between the 4th and 5th ionisation energies shows the 5th electron is in a new, inner shell, confirming 4 outer shell electrons.
Example 4 · Nuclear notation · Paper 1 style
An atom of element Y has the symbol ⁵⁶₂₆Y. State the number of protons, neutrons and electrons in a neutral atom of Y.
  1. Atomic number Z = 26 → number of protons = 26
  2. Neutral atom → number of electrons = number of protons = 26
  3. Number of neutrons = A − Z = 56 − 26 = 30
✓ Answer: Protons = 26, Neutrons = 30, Electrons = 26

Paper 1 · Paper 2 · Command words · Mark scheme language

Exam Technique

DO — Mark scheme habits
  • State the full equation for 1st IE: X(g) → X⁺(g) + e⁻ with state symbols
  • Use "per mole" language when defining IE: "energy to remove one mole of electrons from one mole of gaseous atoms"
  • Show all working in Aᵣ calculations — method marks are available even for a wrong answer
  • Reference "greater nuclear attraction" or "less shielding" when explaining IE trends — vague answers score 0
  • For electron config of ions, remove electrons from highest energy subshell first (4s before 3d)
DON'T — Common errors
  • Don't write Cu as [Ar] 3d⁹ 4s² — the exception is [Ar] 3d¹⁰ 4s¹
  • Don't omit state symbols in ionisation energy equations
  • Don't confuse mass number (A) with atomic mass (Aᵣ) — they are different quantities
  • Don't say "more electrons" when explaining why IE decreases down a group — specify shielding and distance from nucleus
  • Don't use "shells" when "subshells" or "orbitals" is needed for full marks

Key command words for this topic

State
Give a concise answer — no explanation needed. e.g. "State the number of neutrons in ⁵⁶Fe."
Define
Give the precise meaning of a term. Learn the syllabus wording for 1st IE — examiners compare against it.
Explain
Give a reason using chemistry. A bare trend without a cause scores 0. Always reference particles and forces.
Deduce
Use the data given to reach a conclusion. In IE graphs, identify the jump and link it to shell structure.
Paper 2 data question — successive IE graphs: These appear almost every year. Practise: (1) identifying the big jump, (2) stating how many outer electrons this implies, (3) identifying the group. This earns 2–3 marks and takes under 2 minutes with practice.

High-mark definition: First Ionisation Energy

Full marks definition (memorise this): "The energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous singly-charged positive ions."

Equation (must include state symbols): X(g) → X⁺(g) + e⁻

Common mark scheme catches: missing "gaseous", missing "per mole", missing state symbols, or writing it as an exothermic process (it is always endothermic — ΔH is positive).

All learning outcomes from the 9701 syllabus (2025–2027) · tick off as you revise

Syllabus Checklist

Use this checklist against the official Cambridge 9701 syllabus document. Each item below is a learning outcome — examiners write questions to test these exact statements. If you can't do an item confidently, go back to the Notes tab.

1.1 Subatomic particles & nuclear notation

  • Identify and describe protons, neutrons and electrons in terms of relative charge and relative mass
  • Deduce the number of protons, neutrons and electrons in atoms and ions given the atomic number, mass number and charge
  • Describe the distribution of mass and charge within an atom
  • Define the term isotope

1.2 Relative atomic mass & mass spectrometry

  • Define the terms relative isotopic mass and relative atomic mass (in terms of ¹²C)
  • Describe the operation of a mass spectrometer (ionisation, acceleration, deflection, detection)
  • Interpret a simple mass spectrum to calculate relative atomic mass
  • Calculate Aᵣ from % abundance and isotopic masses

1.3 Electron configuration

  • Describe the number of electrons in s, p and d subshells and in s, p and d orbitals
  • Describe the shapes of s and p orbitals
  • State the Aufbau principle, Hund's rule and the Pauli exclusion principle
  • Write full electron configurations for atoms and ions (including transition metals)
  • Explain the anomalous configurations of Cr and Cu
  • Write abbreviated (noble gas core) electron configurations

1.4 Ionisation energy

  • Define first ionisation energy; write and interpret the equation for 1st IE
  • Explain the general trend in first ionisation energies across Period 2 and Period 3
  • Explain the dip in 1st IE between Groups 2→3 (new subshell) and 5→6 (electron pairing)
  • Explain the trend in first ionisation energies down a group
  • Explain how successive ionisation energies provide evidence for shell structure
  • Deduce the group of an element from its successive ionisation energy data
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