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Cambridge IGCSE Physics · 0625

Chapter 5: Nuclear physics — Part 2

Topic 5.1.2 · The nucleus

Protons, neutrons and nuclide notation

The nucleus contains protons and neutrons.

ParticleRelative charge
Proton+1
Neutron0
Electron−1

Proton number (atomic number) Z is the number of protons. Nucleon number (mass number) A is the number of protons plus neutrons.

Number of neutrons = A − Z.

Nuclide notation: AZX .

The relative charge of a nucleus is equal to Z (in units of the proton charge). The relative mass of a nucleus is equal to A (nucleon units).

Nuclide notation A over Z before the symbol X, with A = protons + neutrons, Z = proton number, and two isotopes of the same element shown with the same Z and different A.
Diagram 1: Nuclide notation A over Z before the symbol X, with A = protons + neutrons, Z = proton number, and two isotopes of the same element shown with the same Z and different A.

Isotopes, fission and fusion

Isotopes are nuclei of the same element (same Z) with different A (different numbers of neutrons). An element can have more than one isotope.

Nuclear fission is the splitting of a nucleus. Nuclear fusion is the joining of nuclei. Both can be written as a nuclide equation. A small decrease in mass is released as energy (no numerical mass–energy values are required).

Nuclear fission as a heavy nucleus splitting, and nuclear fusion as light nuclei joining, both with a small mass decrease releasing energy (no numerical values required).
Diagram 2: Nuclear fission as a heavy nucleus splitting, and nuclear fusion as light nuclei joining, both with a small mass decrease releasing energy (no numerical values required).

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