The nuclear model of the atom
Protons, neutrons and electrons, nuclide notation, isotopes and the alpha-scattering evidence.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 5.1.1Describe the structure of the atom in terms of a nucleus containing protons and neutrons, surrounded by electrons.
- 5.1.2Use nuclide notation and define proton number and nucleon number.
- 5.1.3Describe how the alpha-particle scattering experiment supports the nuclear model.Supplement
6 minute read
The nuclear model of the atom
An atom has a tiny, dense, positively charged nucleus at its centre, containing protons and neutrons, surrounded by electrons occupying shells at relatively large distances from the nucleus. Almost all the mass of an atom is in the nucleus; almost all of its volume is empty space occupied by the electron shells.
Nuclide notation
An atom is written as ᴬZX, where A is the nucleon number (protons + neutrons) at top left, and Z is the proton number (number of protons) at bottom left.
- Proton number identifies the element.
- Neutron number = nucleon number − proton number.
- In a neutral atom, number of electrons = number of protons.
Evidence for the nuclear model: alpha-particle scattering
A thin gold foil was bombarded with alpha particles. The results:
- Most alpha particles passed straight through with little or no deflection — showing that the atom is mostly empty space.
- A small number were deflected through large angles, and a very few bounced almost straight back — showing that there is a small, dense region at the centre (the nucleus), and that it is positively charged, since it repelled the positive alpha particles.
This experiment replaced an earlier "plum pudding" model, in which positive charge was thought to be spread evenly throughout the atom, with the modern nuclear model.
Think of it like this
If an atom were scaled up so its nucleus were the size of a marble, the electron shells would extend out to roughly the size of a football stadium — which is exactly why almost every alpha particle in the scattering experiment sailed straight through undisturbed, and only the rare direct approach to the tiny nucleus was deflected.
Worked examples
Method, step by step
An atom is represented as ²⁷₁₃Al. State the number of protons, neutrons and electrons.
- 1Proton number = 13, so protons = 13
- 2Neutrons = nucleon number − proton number = 27 − 13 = 14
- 3Neutral atom, so electrons = protons = 13
13 protons, 14 neutrons, 13 electrons
Common misconceptions
- Believing electrons orbit the nucleus like planets around the Sun in fixed, visible paths. They occupy shells at characteristic energies; the "orbit" picture is a simplification.
- Thinking the alpha-scattering experiment disproved that atoms have any structure at all. It specifically disproved the idea of evenly spread positive charge, replacing it with a concentrated positive nucleus.
- Confusing proton number with nucleon number — proton number identifies the element; nucleon number also includes the neutrons.
In the exam
- When explaining the scattering experiment, link each observation to its conclusion explicitly: "most passed through, so..." and "a few deflected, so..." — do not just state the conclusions on their own.
- Nuclide notation questions often test whether you can extract protons, neutrons and electrons correctly — write out all three explicitly rather than trying to answer from memory.