If an atom were the size of a football stadium, its centre would be the size of a single small marble resting on the halfway line. This vast difference in scale highlights that an atom is mostly empty space.
The overall diameter of an atom is approximately metres. In contrast, the tiny, dense, positively charged centre, known as the nucleus, is approximately metres across. This makes the nucleus between and times the size of the entire atom. Almost all the mass of the atom is concentrated in this tiny central nucleus. Negatively charged subatomic particles called electrons orbit at specific distances through the surrounding empty space.
Scientific theories are never entirely finished—they constantly evolve as new technology uncovers fresh experimental evidence. When old models cannot explain new observations, they must be discarded or updated.
To understand why a metal spoon feels solid, it is strange to realise that the atoms making it up are almost entirely empty. This fact was discovered by Ernest Rutherford, Hans Geiger, and Ernest Marsden using a clever experimental setup.
They fired a beam of alpha particles (positively charged helium nuclei containing two protons and two neutrons, ) at a target of thin gold foil. Gold was chosen due to its extreme malleability, allowing it to be hammered until it was roughly 400 nm thick (only a few atoms wide). The experiment was conducted in a vacuum to prevent the alpha particles from being absorbed or scattered by air molecules. A zinc sulfide screen surrounded the foil, producing a tiny flash of light, or scintillation, whenever an alpha particle hit it.
Scientists expected the alpha particles to pass straight through the gold foil with minimal deflection, assuming the positive charge in the gold atoms was thinly spread out as predicted by the Plum Pudding model. Instead, they observed three distinct phenomena:
| Experimental Observation | Corresponding Scientific Conclusion |
|---|---|
| Most alpha particles (~99%) passed straight through the foil. | The atom is mostly empty space. |
| Some particles were deflected at small angles. | There is a concentrated positive charge in the centre of the atom that repelled the positive alpha particles (electrostatic repulsion). |
| A very small number (approx. 1 in 8,000) bounced straight back at angles . | The mass is concentrated in a tiny, dense nucleus. |
Rutherford's model had a flaw: classical physics predicted that orbiting electrons would constantly lose energy and quickly spiral into the nucleus, causing the atom to collapse. Niels Bohr solved this by proposing that electrons exist in fixed orbits, or shells, and cannot exist in the space between them.
Bohr's model successfully explained how atoms interact with electromagnetic radiation. When an electron absorbs electromagnetic radiation, it gains energy and moves to a higher energy level further from the nucleus, entering an excited state.
Conversely, when an electron moves closer to the nucleus to a lower energy level (such as the ground state), it emits electromagnetic radiation. Bohr's theoretical calculations for these energy jumps perfectly matched the experimental light spectra observed from elements, leading to widespread acceptance of his model.
Students often state that the Plum Pudding model contained a nucleus. It did NOT—the positive charge and mass were spread evenly throughout the whole sphere.
In 6-mark questions explaining the alpha scattering experiment, examiners expect you to explicitly pair each observation with its specific conclusion (e.g., passing straight through proves empty space). Do not just list all observations and then all conclusions.
Always use the precise Edexcel terminology 'mass is concentrated in the nucleus' rather than just saying 'the nucleus is heavy'.
If asked why the alpha scattering experiment was done in a vacuum, state clearly that it prevents the alpha particles from colliding with air molecules, which would alter their path or absorb them.
Nucleus
The extremely tiny, dense, positively charged centre of an atom where almost all of its mass is concentrated.
Electron
A negatively charged subatomic particle that orbits the nucleus of an atom at specific distances.
Plum Pudding Model
An obsolete atomic model proposing the atom is a sphere of diffuse positive charge with negative electrons embedded throughout it, with mass evenly distributed.
Nuclear Model
The atomic model featuring a tiny, dense, positively charged central nucleus surrounded by empty space and orbiting electrons.
Energy level
A specific, fixed distance from the nucleus where an electron can orbit without losing energy; also known as an electron shell.
Alpha particle
A positively charged particle consisting of two protons and two neutrons (a helium nucleus) emitted during radioactive decay.
Malleability
The physical property of a metal that allows it to be permanently deformed or hammered into very thin sheets without breaking.
Scintillation
A brief flash of light produced when an ionizing particle, like an alpha particle, strikes a sensitive material.
Excited state
The condition of an atom when an electron has absorbed energy and moved to a higher energy level further from the nucleus.
Ground state
The lowest possible energy level that an electron can occupy within an atom.
Put your knowledge into practice — try past paper questions for Physics
Nucleus
The extremely tiny, dense, positively charged centre of an atom where almost all of its mass is concentrated.
Electron
A negatively charged subatomic particle that orbits the nucleus of an atom at specific distances.
Plum Pudding Model
An obsolete atomic model proposing the atom is a sphere of diffuse positive charge with negative electrons embedded throughout it, with mass evenly distributed.
Nuclear Model
The atomic model featuring a tiny, dense, positively charged central nucleus surrounded by empty space and orbiting electrons.
Energy level
A specific, fixed distance from the nucleus where an electron can orbit without losing energy; also known as an electron shell.
Alpha particle
A positively charged particle consisting of two protons and two neutrons (a helium nucleus) emitted during radioactive decay.
Malleability
The physical property of a metal that allows it to be permanently deformed or hammered into very thin sheets without breaking.
Scintillation
A brief flash of light produced when an ionizing particle, like an alpha particle, strikes a sensitive material.
Excited state
The condition of an atom when an electron has absorbed energy and moved to a higher energy level further from the nucleus.
Ground state
The lowest possible energy level that an electron can occupy within an atom.