AQA GCSE Physics

Revision Notes

4.1.3 Atomic & Mass Number

Atomic & Mass Number

Atomic Number

  • The number of protons in an atom is called its atomic number (it can also be called the proton number)
    • Elements in the periodic table are ordered by their atomic number
    • Therefore, the number of protons determines which element an atom is
  • The atomic number of a particular element is always the same
  • For example:
    • Hydrogen has an atomic number of 1. It always has just one proton
    • Sodium has an atomic number of 11. It has 11 protons
    • Uranium has an atomic number of 92. It has 92 protons
  • The atomic number is also equal to the number of electrons in an atom
    • This is because atoms have the same number of electrons and protons in order to have no overall charge

Mass Number

  • The total number of particles in the nucleus of an atom is called its mass number
  • The mass number is the number of protons and neutrons in the atom
  • The number of neutrons can be found by subtracting the atomic number from the mass number

Number of Neutron = Mass Number – Atomic Number

  • For example, if a sodium atom has a mass number of 23 and an atomic number of 11, then the number of neutrons would be 23 – 11 = 12

Exam Tip

You may have noticed that the number of electrons is not part of the mass number. This is because electrons have a tiny mass compared to neutrons and protons. We say their mass is negligible when compared to the particles in the nucleus.

Nuclear Notation

  • The mass number and atomic number of an atom are shown by writing them with the atomic symbol
    • This is called nuclear notation
  • Here are three examples:

Nuclear Notation, downloadable IGCSE & GCSE Physics revision notes

Examples of nuclear notation for atoms of Hydrogen, Sodium and Uranium

  • The top number is the mass number
    • This is equal to the total number of particles (protons and neutrons) in the nucleus
  • The lower number is the atomic number
    • This is equal to the total number of protons in the nucleus
  • The atomic and mass number of each type of atom in the examples above is shown in this table:

Worked Example

The element symbol for gold is Au. How many protons, neutrons and electrons are in the gold atom?

Gold atom, downloadable IGCSE & GCSE Physics revision notes

Worked example table, downloadable IGCSE & GCSE Physics revision notes

ANSWER:  D

Step 1: Determine the atomic and mass number

    • The gold atom has an atomic number of 79 (lower number) and a mass number of 197 (top number)

Step 2: Determine the number of protons

    • The atomic number is equal to the number of protons
    • The atom has 79 protons

Step 3: Calculate the number of neutrons

    • The mass number is equal to the number of protons and neutrons
    • The number of neutrons is equal to the mass number minus the atomic number

197 – 79 = 118

    • The atom has 118 neutrons

Step 4: Determine the number of electrons

    • An atom has the same number of protons and electrons
    • The atom has 79 electrons

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