Nucleon & Proton Number (Edexcel International A Level Physics)

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Nucleon & Proton Number

  • Atomic symbols are written in a specific notation as shown below:

AZX Notation, downloadable AS & A Level Physics revision notes

Atomic symbols show the proton number and nucleon number

  • The top number A represents the nucleon number or the mass number
    • Nucleon number (A) = total number of protons and neutrons in the nucleus

  • The lower number Z represents the proton or atomic number
    • Proton number (Z) = total number of protons in the nucleus

  • Note: In Chemistry, the nucleon number is referred to as the mass number and the proton number as the atomic number. The periodic table is ordered by atomic number


  • Although all atoms of the same element always have the same number of protons (and hence electrons), the number of neutrons can vary
  • An isotope is an atom (of the same element) that has an equal number of protons but a different number of neutrons
    • For example, hydrogen has two isotopes: deuterium and tritium. Both isotopes have a proton number of 1
    • Deuterium has one neutron, so its nucleon number is 2
    • Tritium has two neutrons, so its nucleon number is 3

Worked example

Exam Tip

Remember which number in the chemical notation is the nucleon number and proton number is vital for many topics involving particle decays e.g. radioactivity. The most common mistake is thinking the nucleon number is the number of neutrons, the number of neutrons is calculated by:

number of neutrons = nucleon number – proton number

In all neutral (uncharged) atoms, the number of protons = number of electrons.

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.