The structure of an atom; Mass number, atomic number and isotopes

SPEC 8463 • MODULE 4 • PAPER 1

Atomic Structure & Isotopes

The structure of an atom, mass number, atomic number, isotopes and the AZX notation. The foundation for everything in Module 4 — radiation, half-life and nuclear equations all build on this.

Why This Lesson Matters

Module 4 (Atomic Structure) appears on every Paper 1 series, and almost every question in it uses the notation, scale and isotope ideas you'll lock in here. Get this lesson right and the rest of the module gets easier.

⭐

High Priority Topic

4.4.1

Spec Reference

P1

Paper 1 (Higher)

By the end of this lesson you can…

Name the parts of an atom and state their relative charges and masses

State the radius of an atom (about 1 × 10⁻¹⁰ m) and that the nuclear radius is less than 1/10,000 of that

Use ᴬZX notation to read off protons, neutrons and electrons

Define an isotope precisely using mark-scheme wording

Explain how electrons move between energy levels by absorbing or emitting EM radiation

The Structure of an Atom

An atom has a tiny dense nucleus at the centre, made of protons and neutrons. Electrons orbit the nucleus in fixed shells (also called energy levels). The example below is a carbon-12 atom — 6 protons, 6 neutrons, 6 electrons (2 in the inner shell, 4 in the outer shell).

Key fact

Almost all the mass of an atom is in the nucleus. The electrons have a relative mass of approximately zero — they barely contribute to the atom's mass.

The Scale of an Atom

The spec gives two facts about size that you need to know exactly — both written in standard form.

1 × 10⁻¹⁰ m

Radius of an atom

< 1/10,000

Nuclear radius (as fraction of atom)

What this means numerically

If the atomic radius is about 1 × 10⁻¹⁰ m, then the nuclear radius is less than 1 × 10⁻¹⁴ m (i.e. 1/10,000 of 10⁻¹⁰). Most of the mass of an atom is concentrated in this tiny nucleus, so the atom is mostly empty space.

Standard form refresher (MS 1b)

The spec requires you to recognise expressions in standard form. 1 × 10⁻¹⁰ m means a decimal point followed by nine zeros and then a 1 — i.e. 0.0000000001 m. Each step down in the negative power makes the number ten times smaller.

Examiner Alert

Stick to the spec figures. Atomic radius = 1 × 10⁻¹⁰ m (not 10⁻¹¹ or 10⁻⁹). For the nucleus, quote the inequality — "less than 1/10,000 of the radius of an atom". Wrong power = wrong answer.

Subatomic Particles

Three particles. Three locations. Three values to memorise for relative mass and relative charge.

ParticleLocationRelative MassRelative Charge

ProtonNucleus1+1

NeutronNucleus10

ElectronEnergy levels (shells)≈ 0 (very small)−1

Memory Hook

Proton = Positive. Neutron = Neutral. Electron is the only one with a negative charge — and it's tiny compared to the other two.

Atoms Are Electrically Neutral

In any neutral atom the number of protons (+) equals the number of electrons (−). The charges cancel exactly, so the atom carries no overall charge.

Why neutral?

Each proton has charge +1. Each electron has charge −1. If they're equal in number, the total charge is zero.

Atoms → positive ions

The physics spec covers one ion-forming process: an atom loses one or more outer electron(s) and becomes a positive ion. It now has more protons than electrons, giving it an overall positive charge.

The number of protons (and so the element itself) never changes when an atom becomes a positive ion — only the electron count drops.

The ᴬZX Notation

Every atom can be written in one compact symbol that tells you exactly how many protons and neutrons it has.

14 6 C

Read as "carbon-14"

A = Mass number (top-left) = total protons + neutrons

Z = Atomic number (bottom-left)

= number of protons

From the symbol you can read off…

Protons = Z (for ¹⁴₆C: 6 protons)

Electrons = Z (in a neutral atom; for ¹⁴₆C: 6 electrons)

Neutrons = A − Z (for ¹⁴₆C: 14 − 6 = 8 neutrons)

Worked Example: Counting Neutrons

An atom of uranium-235 (²³⁵₉₂U) has atomic number 92. Calculate the number of neutrons. 1 mark

E

Number of neutrons = mass number − atomic number = A − Z

S

Substitute: 235 − 92

A

Number of neutrons = 143

(No unit — it's a count, not a measurement.)

Isotopes

Isotope

Atoms of the same element (same number of protons) but with different numbers of neutrons (so different mass numbers).

Same Z. Different A. The three isotopes of carbon are the classic example:

¹²₆C

6 protons

6 neutrons

6 electrons

¹³₆C

6 protons

7 neutrons

6 electrons

¹⁴₆C

6 protons

8 neutrons

6 electrons

Why this matters in physics

All three isotopes of carbon behave identically chemically (same electrons). But the different neutron numbers change the nuclear stability — carbon-12 and carbon-13 are stable, while carbon-14 is radioactive and decays over time. This is the bridge to the rest of Module 4: which isotopes are radioactive, how they decay, and at what rate.

Electron Energy Levels

Electrons don't sit at random distances. They occupy fixed shells (also called energy levels) at specific distances from the nucleus. Movement between levels needs energy.

↑ Absorb EM radiation

An electron absorbs electromagnetic radiation and moves to a higher energy level — further from the nucleus.

↓ Emit EM radiation

An electron drops back down to a lower energy level — closer to the nucleus — and emits electromagnetic radiation as it does.

Examiner Alert

Get the direction the right way round. Absorb = move up. Emit = move down. Examiner reports flag this almost every series — reversing the direction is treated as a physics error and scores zero marks. Many students lose easy marks here.

A Subtle Distinction

One trap shows up in mark schemes year after year: confusing mass number with atomic mass. They're not the same thing.

✗ Wrong

Mass number = "the mass on the periodic table"

Marks lost. The number on the periodic table is the atomic mass — a weighted average across all isotopes.

✓ Correct

Mass number = total number of protons + neutrons in one specific atom.

Always a whole number. Specific to one isotope, not an average.

In the exam

If a question gives you ²³⁵₉₂U, the mass number is exactly 235 — that's an integer count of nucleons. Don't reach for the periodic table's "238.03" — that's an average across all uranium isotopes.

Multi-Step Practice

Two neutral atoms have the following symbols: ²³₁₁Na and ²⁴₁₁Na.

(a) For each atom, state the number of protons, neutrons and electrons. 3 marks

(b) Explain why these two atoms are isotopes of each other. 2 marks

a1

²³₁₁Na: protons = 11, neutrons = 23 − 11 = 12, electrons = 11

a2

²⁴₁₁Na: protons = 11, neutrons = 24 − 11 = 13, electrons = 11

a3

(1 mark for protons in both, 1 mark for neutrons in both, 1 mark for electrons in both — full breakdown required.)

b1

Both atoms have the same number of protons (atomic number Z = 11)

b2

But they have a different number of neutrons (12 vs 13), so different mass numbers. That is the definition of isotopes.

Common Errors — and the Fixes

Pulled directly from AQA examiner reports for Paper 1 atomic-structure questions. Mark these and avoid them.

Students often write…Mark-scheme versionWhy it matters

"Isotopes have different number of protons"Isotopes have the same number of protons but different numbers of neutronsReversed definition — scores 0

"Carbon-12 and carbon-14 differ in neutrons by 12 and 14"Differ by 2 neutrons (8 − 6 = 2)Confuses mass number with neutron count — recurring error

"Atomic radius is 10⁻¹¹ m"Atomic radius is 1 × 10⁻¹⁰ m ; nuclear radius is less than 1/10,000 of thatWrong power = wrong answer

"Electrons absorb EM and move down a level"Electrons absorb EM to move up ; emit EM to move downDirection reversal = physics error, scores 0 (flagged most series)

"An atom gains an electron to become a positive ion"An atom loses an outer electron to become a positive ionSpec specifies "loses … outer electron(s)" only

Stretch Question

Two atoms have the following symbols: ⁴⁰₂₀Ca and ⁴⁰₁₈Ar.

A student says: "These atoms must be isotopes of each other because they have the same mass number."

Is the student correct? Explain your answer. 2 marks

1

The student is incorrect.

2

Isotopes must have the same atomic number (same number of protons / same element). ⁴⁰₂₀Ca has 20 protons (calcium); ⁴⁰₁₈Ar has 18 protons (argon) — they're different elements.

Tip: atoms with the same mass number but different elements are called "isobars" — but that term is not on the GCSE spec. The mark is for spotting that isotopes need same Z, not same A.

Examiner Alerts — The Big Four

Alert 1 — Standard form & scale

Atomic radius = 1 × 10⁻¹⁰ m. Nuclear radius = less than 1/10,000 of the radius of an atom. Always state the unit. Wrong powers of ten lose the mark.

Alert 2 — Isotope definition

"Same number of protons, different number of neutrons." Memorise this exactly. Saying "same atomic number, different mass number" is also creditworthy — but you must say both halves.

Alert 3 — Energy levels direction

Absorb → up. Emit → down. Reverse it and the answer scores zero (physics error). Recurring — flagged almost every series.

Alert 4 — Positive ions only

The physics spec covers only one ion-forming process: an atom loses outer electron(s) to form a positive ion. Don't introduce negative-ion content — it isn't in 4.4.1.2.

Checkpoint — What You Can Now Do

Tick each one off mentally before moving on

✓ Name the parts of an atom and the location of each subatomic particle

✓ State that atomic radius ≈ 1 × 10⁻¹⁰ m and the nuclear radius is less than 1/10,000 of that

✓ State the relative mass and charge of protons, neutrons and electrons

✓ Explain why an atom is electrically neutral and how it becomes a positive ion by losing outer electron(s)

✓ Use ᴬZX notation to find protons, neutrons and electrons

✓ Define an isotope using mark-scheme wording and identify two isotopes of the same element

✓ Describe how electrons absorb or emit EM radiation when moving between energy levels

Next Lesson

Lesson 4.02 — The Development of the Model of the Atom. How scientific evidence changed our picture of the atom from Dalton's sphere to Bohr's energy levels — and why the alpha-scattering experiment was the turning point.