Nuclear Notation (OCR GCSE Combined Science A (Gateway)) : Revision Note

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Nuclear Notation

Properties of Atoms

  • The atom is made up of:

    • Protons - positively charged particles with a relative atomic mass of one unit, found in the nucleus

    • Neutrons – no charge, and also with a relative atomic mass of one unit, found in the nucleus

    • Electrons – negative charge with almost no mass (1/2000 the mass of a proton or neutron)

  • The properties of each of the particles are shown in the table below:

Particles in the atom, downloadable GCSE Physics revision notes
  • Although atoms contain particles of different charge, the total charge within an atom is zero

    • This is because the number of electrons is equal to the number of protons

  • For example, the following table sets out the calculation of the total charge in the Lithium atom:

Charge calculation, downloadable IGCSE & GCSE Physics revision notes

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

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

Examiner Tips and Tricks

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.

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Joanna

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