Atomic Structure: Sub-Atomic Particles and the Development of the Atomic Model

This guide covers subtopic C1.2, Atomic structure, from Topic C1, Particles, of OCR GCSE (9–1) Chemistry A (Gateway Science) J248. It applies to both Foundation Tier (Papers 1 and 2) and Higher Tier (Papers 3 and 4). Some content within this subtopic is restricted to candidates taking the separate sciences rather than Combined Science; check with your teacher or the current specification if you need to know exactly which statements that affects.

Before studying this

Subtopic C1.1, the particle model, introduces the different states of matter and the idea that elements are made of one type of atom while compounds combine atoms of different elements. This page assumes that grounding and moves inside the atom itself: what it’s made of, how the model of it has changed over time, and how to work out its structure from a few given numbers.

Syllabus coverage

OCR GCSE (9–1) CHEMISTRY A (GATEWAY SCIENCE) J248 — Topic C1.2

This subtopic covers: how and why the atomic model has changed over time, from Dalton through to Bohr; describing the atom as a positively charged nucleus surrounded by negatively charged electrons, with a nuclear radius much smaller than the atom’s and almost all of the atom’s mass concentrated in the nucleus; recalling the typical size of atoms and small molecules; recalling the relative charges and approximate relative masses of protons, neutrons and electrons; and calculating the numbers of protons, neutrons and electrons in atoms and ions, given the atomic number and mass number of an isotope.

How the atomic model changed over time

Scientific models are refined as new evidence emerges, and the atomic model is a textbook example of that process:

• Dalton (early 1800s) proposed atoms as solid, indivisible spheres — different for each element, but with nothing smaller inside them.
• Thomson, after discovering the electron, proposed the “plum pudding” model: a ball of positive charge with negative electrons embedded through it.
• Rutherford, interpreting the alpha-particle scattering experiment carried out by Geiger and Marsden, showed that most of an atom’s mass and all of its positive charge are concentrated in a tiny, dense nucleus — overturning the plum pudding model, since a diffuse positive charge could never have deflected alpha particles back the way the experiment showed.
• Bohr proposed that electrons occupy fixed orbits (shells) at specific distances from the nucleus, rather than existing anywhere around it — a model that correctly predicted atomic spectra.

Each step was driven by an experimental result the previous model couldn’t explain — a useful thing to be able to describe, not just the final model itself.

The structure of the atom

A modern atom is described as a positively charged nucleus — containing protons and neutrons — surrounded by negatively charged electrons. Two structural facts matter for calculations and for explaining scattering experiments:

• The nuclear radius is much smaller than the radius of the atom as a whole (roughly 1/10,000th).
• Almost all of the atom’s mass is concentrated in the nucleus, since electrons have a negligible mass compared with protons and neutrons.

Particle	Relative charge	Approximate relative mass
Proton	+1	1
Neutron	0	1
Electron	-1	very small (negligible)

The scale of atoms

Atoms and small molecules have a typical radius on the order of 10⁻¹⁰ metres — a scale worth being able to recall directly, since questions may ask you to compare it with other sizes (a human hair, a bacterium, a nanoparticle) or reason about how many atoms would span a given distance.

Working out atomic structure: atomic number, mass number and isotopes

Two numbers, both usually given or shown in standard notation, are enough to work out the full particle count of any atom or ion:

• Atomic number (Z) — the number of protons. This defines which element it is, and in a neutral atom it also equals the number of electrons.
• Mass number (A) — the total number of protons and neutrons (not electrons, since their mass is negligible).

So for a neutral atom: neutrons = mass number − atomic number, and electrons = atomic number.

Isotopes are atoms of the same element (same atomic number, same number of protons) with a different number of neutrons, and therefore a different mass number.

Worked example. An atom of chlorine-37 has atomic number 17 and mass number 37. How many protons, neutrons and electrons does it have?

• Protons = atomic number = 17
• Neutrons = mass number − atomic number = 37 − 17 = 20
• Electrons = atomic number (neutral atom) = 17

Worked example — an ion. An Mg²⁺ ion has atomic number 12 and mass number 24. How many protons, neutrons and electrons does it have?

• Protons = atomic number = 12 (forming an ion never changes the number of protons)
• Neutrons = mass number − atomic number = 24 − 12 = 12
• Electrons: a neutral magnesium atom has 12 electrons; forming a 2+ ion means losing 2 electrons, so this ion has 12 − 2 = 10 electrons

Common mistakes

• Assuming forming an ion changes the nucleus. A positive ion has lost electrons, not gained protons; a negative ion has gained electrons, not lost protons. The number of protons never changes when an ion forms — that would make it a different element entirely.
• Confusing atomic number and mass number when finding neutrons. Neutrons come from the difference between mass number and atomic number, not either number alone.
• Treating isotopes as having different chemical properties. Isotopes of the same element have the same number of protons and electrons, so they react chemically in the same way — only their mass (and therefore some physical properties) differs.
• Forgetting that most of an atom is empty space. The nucleus is extremely small relative to the atom as a whole — a fact that only makes sense once you know Rutherford’s scattering result showed most alpha particles passing straight through gold foil.

Quick revision checklist

• The sequence of atomic models: Dalton, Thomson, Rutherford (using Geiger and Marsden’s scattering results), Bohr
• Atom structure: positive nucleus (protons + neutrons), negative electrons, tiny nuclear radius relative to the atom, almost all mass in the nucleus
• Typical atomic/small-molecule radius: order of 10⁻¹⁰ m
• Relative charges and masses of protons, neutrons and electrons
• Calculating protons, neutrons and electrons from atomic number, mass number and ion charge

Related resources

• Atomic Structure (Cambridge IGCSE / O Level) — the equivalent subtopic from a different awarding body, useful for comparing how the same core chemistry is structured across specifications
• OCR GCSE Chemistry hub

Written against OCR GCSE (9–1) Chemistry A (Gateway Science) J248, specification version 4.0 (August 2026), https://www.ocr.org.uk/qualifications/gcse/gateway-science-suite-chemistry-a-j248-from-2016/. Always check the current specification for your examination year.