Nucleon & Proton Number (Edexcel International A Level Physics)

Nucleon & Proton Number

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

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

Isotopes

• 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