Refining the atom: Bohr, protons and neutrons Revision Notes | GCSE AQA Higher Chemistry
Niels Bohr suggested electrons orbit the nucleus in fixed energy levels, and James Chadwick discovered the neutron in 1932. Free notes and 9 questions answered.
Updated 19 Mar 2026 · Revise Right Team
Refining the atom: Bohr, protons and neutrons is part of A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes in GCSE AQA Higher Chemistry.
3 min
Reading time
20
Lessons in this topic
GCSE AQA Higher
Pathway
Reviewed by Revise Right
Content review
What this lesson covers
Niels Bohr suggested electrons orbit the nucleus in fixed energy levels, and James Chadwick discovered the neutron in 1932. Free notes and 9 questions answered.
Describe how Bohr refined the nuclear model by proposing electrons orbit at specific distances
Explain how experimental evidence led to the discovery of protons within the nucleus
Evaluate Chadwick's contribution to atomic theory through the discovery of neutrons
Compare the nuclear model before and after the discoveries of protons and neutrons
Revise Right Mocks '26
AQA GCSE Chemistry mock draft, 26–31 Oct. Draft coverage: Chemistry Paper 1 (Higher tier). Reviewed for an original-practice sitting; check student availability before registering. Marking and results will follow later, after separate approval. Eligible new accounts receive a five-day Pro trial when signup email verification completes. Previous trials keep their original expiry. After expiry, new papers need Pro unless you have an explicitly confirmed free sitting offer for this event. That offer adds no Pro time or AI learning. Saved attempts and released results remain readable. No card or automatic charge.
Use this lesson to stay inside A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes while connecting the detail back to the wider Topic 1: Atomic structure and the periodic table topic.
Topic 1: Atomic structure and the periodic table
A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes
GCSE AQA Higher Chemistry
Revision notes
Read the core explanations from Refining the atom: Bohr, protons and neutrons before testing yourself from memory.
1. The Problem with Rutherford's Model
Rutherford's nuclear model had a major flaw. According to physics, orbiting electrons should spiral into the nucleus and atoms should collapse. But atoms are stable - they don't fall apart.
This is where Niels Bohr stepped in with a revolutionary idea. What if electrons don't orbit randomly, but at specific distances from the nucleus?
2. Bohr's Breakthrough
Bohr suggested electrons orbit at fixed distances - like planets in set orbits around the sun. Electrons can't exist between these orbits, only in them.
The brilliant part? Bohr's mathematical calculations matched experimental observations perfectly. When scientists heated elements and looked at the light they gave off, the patterns matched Bohr's predictions exactly.
3. Discovering Protons
Scientists knew the nucleus was positively charged, but what was it made of? Further experiments revealed something crucial - the positive charge could be broken down into smaller, identical pieces.
Think of it like discovering that a £10 note is made up of ten £1 coins. The nucleus wasn't just 'positive' - it contained whole numbers of identical positive particles called protons.
This explained why different elements had different amounts of positive charge. Hydrogen has 1 proton, helium has 2, lithium has 3, and so on.
4. Chadwick and the Missing Neutron
By the 1920s, scientists had a problem. The mass of atoms didn't match the number of protons. Helium has 2 protons but the mass of 4 protons. Where was the missing mass?
James Chadwick solved this puzzle in 1932 - about 20 years after the nucleus was accepted. His experiments provided evidence for neutral particles in the nucleus with roughly the same mass as protons.
Neutrons explained the missing mass perfectly. Helium has 2 protons + 2 neutrons = mass of 4. Carbon has 6 protons + 6 neutrons = mass of 12.
5. The Complete Picture
By 1932, the atomic model was complete with three particles: protons and neutrons in the nucleus, electrons in fixed orbits around it.
Worked example: Building an Atomic Model Timeline
Arrange the key discoveries in chronological order and explain what each added to atomic theory.
~1911: Nucleus discovered — Atoms have a tiny, dense, positive nucleus with electrons around it
~1913: Fixed electron orbits proposed — Electrons orbit at specific distances, explaining atomic stability
1932: Neutral particles found — Neutrons in nucleus explain atomic mass
Answer: The modern atomic model emerged through four key discoveries over 20 years
6. Exam Technique
Questions about atomic model development often ask you to describe improvements or explain why evidence supported new ideas.
Exam precision note: explain what each discovery added to the model of the atom. Bohr explained why electrons occupy fixed shells, the proton explained the positive charge of the nucleus, and the neutron explained the extra mass of atoms.
When you work out the overall charge of an atom, you must count both protons and electrons. A neutral atom has equal numbers of protons and electrons, so its overall charge is zero even if neutrons are present in the nucleus.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-19.
James Chadwick discovered the neutron in 1932, about 20 years after the nucleus was accepted. His experiments provided evidence for neutral particles in the nucleus with roughly the same mass as protons, which explained the missing mass of atoms.
How did Bohr improve the model of the atom?
Niels Bohr suggested that electrons orbit the nucleus at specific, fixed distances called energy levels or electron shells, and cannot exist between them. This explained why atoms are stable rather than collapsing, and his calculations matched the patterns of light given off by heated elements.
What is a neutron?
A neutron is a neutral particle found in the nucleus with approximately the same mass as a proton but no electrical charge. Neutrons explained the missing mass of atoms that protons alone could not explain.
How were protons discovered?
Further experiments showed that the positive charge of the nucleus could be broken down into smaller, identical particles called protons, each with a charge of +1. This explained why different elements have different amounts of positive charge: hydrogen has 1 proton, helium 2 and lithium 3.
What was the problem with Rutherford's nuclear model?
The problem with Rutherford's nuclear model was that, according to physics, orbiting electrons should spiral into the nucleus and atoms should collapse. But atoms are stable, which Bohr explained with fixed electron orbits.
Why were neutrons needed to explain atomic mass?
The mass of atoms did not match the number of protons: helium has 2 protons but the mass of 4 protons. Neutrons explained the missing mass, since helium has 2 protons + 2 neutrons = mass of 4 and carbon has 6 protons + 6 neutrons = mass of 12.
What is the order of discoveries in the model of the atom?
The model of the atom developed through Rutherford's nucleus (~1911), Bohr's fixed electron orbits (~1913), the discovery that positive charge is made of protons (~1920s) and Chadwick's neutrons (1932), giving protons and neutrons in the nucleus and electrons in fixed orbits.
Why does helium have a mass of 4 but only 2 protons?
Helium has a mass of 4 but only 2 protons because its nucleus also contains 2 neutrons, which have about the same mass as protons. Neutrons explained this missing mass; carbon has 6 protons and 6 neutrons, giving a mass of 12.
What did each discovery add to the model of the atom?
Rutherford's nuclear model added a tiny, dense, positive nucleus; Bohr added fixed electron orbits, explaining stability; the discovery of protons showed the positive charge is made of identical particles; and Chadwick's neutrons explained the extra mass of atoms.