Mass Number, Atomic Number and Isotopes

This lesson covers mass number, atomic number and isotopes as required by the AQA GCSE Physics specification (4.4.1). Understanding these concepts is essential for interpreting the periodic table, writing nuclear equations, and explaining radioactive decay processes.

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Atomic Number (Z)

The atomic number (also called the proton number) is the number of protons in the nucleus of an atom. It is given the symbol Z.

Key facts:

• The atomic number defines the element. All atoms of the same element have the same number of protons.
• For example, every carbon atom has 6 protons (Z = 6). Every uranium atom has 92 protons (Z = 92).
• If you change the number of protons, you change the element.
• In a neutral atom, the atomic number also tells you the number of electrons, because the number of protons equals the number of electrons.
• On the periodic table, elements are arranged in order of increasing atomic number.