top of page

7.2 The Discovery of the Nucleus

GCSE Physics · AQA · Chapter 7: Radioactivity

A free GCSE Physics lesson for AQA (8463) from OBA School: how evidence from alpha scattering replaced the plum pudding model, and how Bohr and Chadwick added to the nuclear model. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the plum pudding model and the nuclear model of the atom
  • Describe the alpha particle scattering experiment, its results and what they showed
  • Explain why the nuclear model replaced the plum pudding model
  • Describe how Bohr’s energy levels and Chadwick’s discovery of the neutron changed the model
  • Compare the sizes of an atom and its nucleus using standard form

Key words

  • Scientific model: A simplified picture or idea used to explain observations and make predictions. It can change when new evidence is found.
  • Plum pudding model: An early model: the atom is a ball of positive charge with negative electrons spread through it.
  • Alpha scattering: An experiment in which alpha particles were fired at thin gold foil to find out what is inside atoms.
  • Nuclear model: The atom has a tiny, positive, dense nucleus with electrons around it; most of the atom is empty space.
  • Energy level: One of the fixed distances (with a fixed energy) at which an electron can orbit the nucleus.
  • Neutron: An uncharged particle in the nucleus with about the same mass as a proton.

Before the nucleus: the plum pudding model

Long ago atoms were thought to be tiny solid spheres that could not be divided. When the electron was discovered (1897), scientists knew atoms contain tiny negative particles. As atoms are neutral, the rest of the atom must be positive. This led to the plum pudding model: a ball of spread-out positive charge with electrons dotted through it.

Key fact: In the plum pudding model the positive charge and the mass are spread evenly through the atom. There is no nucleus and no empty space.

The alpha scattering experiment

Geiger and Marsden, working with Rutherford, fired a narrow beam of alpha particles (small, fast and positive) at a very thin gold foil. The apparatus was in a vacuum so the alpha particles were not stopped by air. A detector was moved around the foil to count how many alpha particles came off at each angle.

  • Most alpha particles went straight through the foil.; What it showed Most of the atom is empty space.
  • A few were deflected (changed direction) by small angles.; What it showed There is a concentrated positive charge that repels the positive alpha particles.
  • A very small number (about 1 in 10 000) bounced back by more than 90°.; What it showed The positive charge and most of the mass are packed into a very small nucleus.

The plum pudding model could not explain the large deflections: spread-out positive charge is far too weak to turn an alpha particle back. Rutherford used the results to propose the nuclear model in 1911.

Exam tip: In an exam, link each result to its conclusion: straight through → empty space; deflected → positive charge; bounced back → tiny nucleus containing most of the mass.

The nuclear model and the size of the nucleus

In the nuclear model the atom has a tiny, positively charged nucleus where almost all the mass is, with electrons around it. Rutherford’s model was accepted because its predictions matched the numbers of alpha particles counted at each angle, and it later helped explain radioactivity as changes in an unstable nucleus.

The radius of an atom is about 1 × 10−¹⁰ m. The radius of a nucleus is less than 1/10 000 of the radius of the atom (about 10−¹⁴ m to 10−¹⁵ m).

Worked example: how many times bigger?

An atom has a radius of 1.4 × 10−¹⁰ m. Its nucleus has a radius of 7.0 × 10−¹⁵ m.
ratio = 1.4 × 10−¹⁰ ÷ 7.0 × 10−¹⁵ = 2.0 × 10⁴
In a scale model with a nucleus of radius 3 mm, the atom would have radius 3 mm × 20 000 = 60 000 mm = 60 m.

Answer: the atom is 20 000 times wider than its nucleus

Exam tip: Divide standard form numbers on a calculator using the ×10ˣ (or EXP) key, and check the answer is sensible: the atom must come out much bigger than the nucleus.

Bohr: electrons in energy levels

Niels Bohr (1913) suggested that electrons orbit the nucleus only at fixed distances, called energy levels. His calculations agreed with the colours of light that atoms are seen to give out.

  • An electron that absorbs electromagnetic radiation moves to a higher energy level, further from the nucleus.
  • An electron that emits electromagnetic radiation moves to a lower energy level, closer to the nucleus.
  • If an atom loses an outer electron completely it becomes a positive ion.

Key fact: Only certain energy jumps are possible, so each element gives out its own set of light frequencies (a “fingerprint” used to identify elements in stars).

Protons, neutrons and changing models

Later experiments showed that the positive charge of any nucleus is a whole number of equal smaller charges. The smallest, the hydrogen nucleus, was named the proton. But most nuclei had more mass than their protons alone could explain, so scientists predicted a neutral particle with about the same mass as a proton. James Chadwick found the evidence for the neutron in 1932, about 20 years after the nucleus was discovered.

  • 1897; Discovery the electron; Model of the atom plum pudding (1904)
  • 1909–1911; Discovery alpha scattering; Model of the atom nuclear model
  • 1913; Discovery electrons in fixed orbits; Model of the atom Bohr’s energy levels
  • about 1920; Discovery protons in the nucleus; Model of the atom nucleus made of protons
  • 1932; Discovery the neutron (Chadwick); Model of the atom protons and neutrons in the nucleus

Exam tip: Models change when new experimental evidence cannot be explained by the old model. A new model is accepted when it explains the evidence and its predictions are tested and confirmed by other scientists.

Common mistakes

  • Saying alpha particles “hit” the nucleus. Most bounce back because they are repelled by the positive charge; they never touch it.
  • Forgetting that most alpha particles went straight through – this is the evidence for empty space.
  • Saying the plum pudding model had a nucleus. It did not: the positive charge was spread through the whole atom.
  • Mixing up which way electrons move: absorbing radiation moves an electron further from the nucleus.
  • Dividing standard form the wrong way round: the atom radius goes on top when asked how many times bigger the atom is.

Revision questions

Try these, then check your answers below each one.

Question 1. Which statement describes the plum pudding model of the atom? (Choose from: A ball of positive charge with electrons spread through it, A tiny positive nucleus with electrons in energy levels, A solid sphere that cannot be divided, A nucleus of protons and neutrons with no electrons.)

Answer: A ball of positive charge with electrons spread through it

Working: In the plum pudding model, positive charge is spread through the whole atom with negative electrons dotted through it. There is no nucleus.

Question 2. In the alpha scattering experiment most alpha particles went straight through the gold foil. What did this show? (Choose from: Most of the atom is empty space, The nucleus is negatively charged, Gold atoms have no electrons, Alpha particles have no charge.)

Answer: Most of the atom is empty space

Working: Most alpha particles met nothing that could deflect them, so most of the atom is empty space.

Question 3. An atom has a radius of 1.2 × 10−¹⁰ m. Its nucleus has a radius of 2.4 × 10−¹⁵ m.<br/>How many times larger is the radius of the atom than the radius of the nucleus?

Answer: 50000

Working: 1.2 × 10−¹⁰ ÷ 2.4 × 10−¹⁵ = 5.0 × 10⁴ = 50 000 times.

Related lessons

Continue in the full course

This lesson is section 7.2 (AGCPH0702) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

bottom of page