Revision Notes

5.1.2 Nucleus

Protons & Neutrons

  • Atoms are made up of three different particles:


atom-proton-neutron-electron, IGCSE & GCSE Physics revision notes

Protons and neutrons are found in the nucleus of an atom


The properties of each of these particles is shown in the table below:

Nucleus table, IGCSE & GCSE Physics revision notes

  • The atom shown in the above diagram can also be represented using an atomic symbol:


Atomic symbol, IGCSE & GCSE Physics revision notes

Atomic symbols, like the one above, describe the constituents of nuclei


  • The top number is called the nucleon number, A, and is equal to the total number of particles (protons and neutrons) in the nucleus
  • The lower number is called the proton number, Z, and is equal to the total number of protons in the nucleus

(Note: Chemists refer to the nucleon number as the mass number, and the proton number as the atomic number)

  • When given an atomic symbol, you can figure out the total number of protons, neutrons and electrons in the atom:
    • Protons: The number of protons is equal to the proton number
    • Electrons: Atoms are neutral, and so in a neutral atom the number of negative electrons must be equal to the number of positive protons
    • Neutrons: The number of neutrons can be found by subtracting the proton number from the nucleon number
  • The term nucleon is used to mean a particle in the nucleus – ie. either a proton or a neutron
  • The term nuclide is used to refer to a nucleus with a specific combination of protons and neutrons



  • Although all atoms of the same element always have the same number of protons (and hence electrons), the number of neutrons can vary:


Three isotopes of hydrogen, IGCSE & GCSE Physics revision notes

The three atoms shown above are all forms of hydrogen, but they each have different numbers of neutrons


  • The number of neutrons in an atom does not affect the chemical properties of an atom, only its mass. Such atoms are called isotopes:
    Isotopes are atoms (of the same element) that have equal numbers of protons but different numbers of neutrons


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