The NucleusCambridge IGCSE Physics: Revision notes
Section 1
Structure of the atom
An atom consists of a tiny, positively charged nucleus at its centre, surrounded by negatively charged electrons in orbit around it. The nucleus itself is made up of protons and neutrons.
Atoms can become ions by losing or gaining electrons: losing electrons leaves a positive ion, gaining electrons leaves a negative ion.
Section 2
Evidence for the nuclear model: alpha particle scattering
The scattering of alpha particles by a thin sheet of metal provided the key evidence for the nuclear model of the atom. Key observations and what they showed:
| Observation | What it showed |
|---|---|
| Most alpha particles passed straight through | The atom is mostly empty space |
| A few were deflected at large angles, some straight back | The nucleus is very small but concentrates most of the atom's mass |
| The deflection was caused by repulsion | The nucleus is positively charged |
This replaced the earlier idea of charge and mass being spread evenly through the atom.
Link each observation directly to what it proves: straight-through particles → mostly empty space; large-angle deflections → small, dense, positive nucleus.
Section 3
Protons, neutrons and electrons
The three particles that make up an atom have specific relative charges:
| Particle | Relative charge | Location |
|---|---|---|
| Proton | +1 | Nucleus |
| Neutron | 0 | Nucleus |
| Electron | −1 | Orbiting the nucleus |
An atom with no overall charge has equal numbers of protons and electrons.
Section 4
Proton number, nucleon number and isotopes
Two numbers describe the composition of a nucleus:
- Proton number (atomic number), Z — the number of protons in the nucleus, and it determines the relative charge on the nucleus
- Nucleon number (mass number), A — the total number of protons and neutrons, and it determines the relative mass of the nucleus
The number of neutrons can be calculated as A − Z.
Atoms are represented using nuclide notation: , where X is the element symbol.
An isotope is a form of an element with the same proton number but a different number of neutrons (and therefore a different nucleon number). An element may have more than one isotope.
Carbon-12 and carbon-14 both have 6 protons (Z = 6) but different numbers of neutrons (6 and 8), making them isotopes of carbon.
Section 5
Fission and fusion: splitting and joining nuclei
Two processes change the nucleus itself:
- Nuclear fission — a large nucleus splits into smaller nuclei
- Nuclear fusion — two smaller nuclei join to form a larger nucleus
Both can be represented with nuclide equations, and both involve mass and energy changes (qualitatively, without needing to calculate exact values at this level).
Must Know
- An atom has a small, positive nucleus (protons + neutrons) surrounded by orbiting electrons
- Relative charges: proton +1, neutron 0, electron −1
- Alpha particle scattering showed the nucleus is small, dense, and positively charged, with the rest of the atom mostly empty space
- Proton number (Z) = number of protons; nucleon number (A) = protons + neutrons
- Isotopes have the same proton number but different neutron numbers
- Fission splits a large nucleus; fusion joins smaller nuclei together
That's the notes covered.
Carry on to the next subtopic.