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Topic 5 · 5.1

The nuclear model of the atom

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe the atom as a positive nucleus with negative electrons in orbit; describe how atoms form positive and negative ions.
  • Describe the nucleus in terms of protons and neutrons, with their relative charges; define proton number and nucleon number; use nuclide notation.
  • Explain what isotopes are.
  • Describe how alpha-particle scattering supports the nuclear model EXTENDED.
  • Describe fission and fusion with nuclide equations; relate proton number to nuclear charge and nucleon number to nuclear mass EXTENDED.

📚The physics

The atom

A beryllium-9 atom: a tiny nucleus of 4 protons and 5 neutrons with 4 electrons orbiting it. A key gives the proton charge +1 and relative mass 1, the neutron charge 0 and relative mass 1, and the electron charge -1 and a mass about 1/1840. The nuclide notation for beryllium-9 has the nucleon number 9 at the top left and the proton number 4 at the bottom left; neutrons = 9 - 4 = 5.
Not to scale: the nucleus is about 100 000 times smaller than the atom.

An atom has a small, positively charged nucleus of protons and neutrons, surrounded by negatively charged electrons in orbit. A neutral atom has equal numbers of protons and electrons. An atom that loses electrons becomes a positive ion; one that gains electrons becomes a negative ion.

proton number (atomic number) Z: the number of protons in the nucleus
nucleon number (mass number) A: the number of protons plus neutrons
number of neutrons = A − Z
EXTENDED the relative charge on a nucleus is +Z; its relative mass is about A

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. An element may have more than one isotope; they behave identically in chemical reactions but may differ in whether they are radioactive.

Three isotopes of hydrogen, each with one proton and one electron: hydrogen-1 with no neutrons, hydrogen-2 or deuterium with one neutron, and hydrogen-3 or tritium with two neutrons.
Same proton number, different nucleon numbers.

Evidence for the nucleus EXTENDED

Alpha particle paths, calculated for an inverse-square repulsion from a gold nucleus, as they pass through thin gold foil. Most travel almost straight through; those passing close to the nucleus are deflected through large angles; one aimed almost straight at it bounces back.
EXTENDED Paths computed for an inverse-square repulsion (the nucleus is drawn far larger than to scale).
  • EXTENDED Most alpha particles pass straight through: the atom is mostly empty space with a very small nucleus.
  • EXTENDED Some are deflected: the nucleus is positively charged (it repels the positive alpha particles).
  • EXTENDED A very few bounce back: the nucleus contains most of the mass of the atom.

Fission and fusion EXTENDED

Fission: a neutron hits a uranium-235 nucleus, which splits into two smaller nuclei, releasing 2 or 3 neutrons and energy; the neutrons can cause a chain reaction. Fusion: a hydrogen-2 and a hydrogen-3 nucleus join to form helium-4 and a neutron, releasing energy, as in the Sun. In both, the total mass after is slightly less than before.
EXTENDED Nuclear power stations use fission; stars are powered by fusion.
\[ {}^{235}_{92}\mathrm{U} + {}^{1}_{0}\mathrm{n} \rightarrow {}^{144}_{56}\mathrm{Ba} + {}^{89}_{36}\mathrm{Kr} + 3\,{}^{1}_{0}\mathrm{n} \]
\[ {}^{2}_{1}\mathrm{H} + {}^{3}_{1}\mathrm{H} \rightarrow {}^{4}_{2}\mathrm{He} + {}^{1}_{0}\mathrm{n} \]

EXTENDED In every nuclide equation the nucleon numbers and the proton numbers balance. Fission is the splitting of a heavy nucleus (after absorbing a neutron) into two smaller nuclei, with neutrons and energy released. Fusion is the joining of two light nuclei into a heavier one, releasing energy. In both, the mass of the products is slightly less than the mass before; the difference has been released as energy.

✏️Worked example

Chlorine has two stable isotopes, \( {}^{35}_{17}\mathrm{Cl} \) and \( {}^{37}_{17}\mathrm{Cl} \). (a) State the numbers of protons, neutrons and electrons in a neutral atom of chlorine-37. [3] (b) State one similarity and one difference between the nuclei of the two isotopes. [2] (c) A chlorine atom gains one electron. Name the particle formed and state its charge. [2]

(a) Protons 17; neutrons 37 − 17 = 20; electrons 17.

(b) Same number of protons (17); different number of neutrons (18 and 20).

(c) A negative ion, with a charge of −1.

Check it. Protons + neutrons must add up to the nucleon number: 17 + 20 = 37. ✓
“Isotopes have different numbers of protons.” Then they would be different elements. Isotopes differ only in neutrons.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Theory.) The nuclide notation for an isotope of sodium is \( {}^{23}_{11}\mathrm{Na} \). Determine the numbers of protons and neutrons in its nucleus. [2] (Modelled on 0625/32 June 2026 Q9(b).)
Protons 11; neutrons 23 − 11 = 12.
2. (Multiple choice.) How does an atom become a positive ion? A: it gains protons. B: it gains electrons. C: it loses electrons. D: it loses neutrons.
C.
3. (Theory.) State the relative charges of the proton, the neutron and the electron. [1]
+1, 0, −1.
4. (Theory.) EXTENDED Describe the results of the alpha-scattering experiment and what each shows about the atom. [3]
Most alpha particles pass straight through the gold foil: the atom is mostly empty space. Some are deflected: the nucleus is positively charged. A very few bounce back: nearly all the mass is concentrated in a very small nucleus.
5. (Theory.) EXTENDED Describe what happens when heavy nuclei undergo fission in a nuclear reactor. [3] (Modelled on 0625/42 June 2026 Q10(a)(i).)
A (slow) neutron is absorbed by a heavy nucleus such as uranium-235; the nucleus splits into two smaller (daughter) nuclei; two or three neutrons and a large amount of energy are released (the neutrons can go on to cause further fissions).
6. (Theory.) EXTENDED Complete the equation: \( {}^{235}_{92}\mathrm{U} + {}^{1}_{0}\mathrm{n} \rightarrow {}^{141}_{56}\mathrm{Ba} + {}^{A}_{Z}\mathrm{Kr} + 3\,{}^{1}_{0}\mathrm{n} \). [2]
Nucleon numbers: 236 = 141 + A + 3, so A = 92. Proton numbers: 92 = 56 + Z, so Z = 36.
7. (Theory.) EXTENDED State one difference between nuclear fission and nuclear fusion. [1]
Fission splits a heavy nucleus into smaller ones; fusion joins light nuclei into a heavier one.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • PhET “Rutherford Scattering” and “Build an Atom”
  • PhET “Nuclear Fission” — chain reactions