Mass number, atomic number and isotopes — AQA GCSE Physics
Test yourself on Mass number, atomic number and isotopes with AQA GCSE practice questions.
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Mass number, atomic number and isotopes explained
A nuclide symbol carries three pieces of information: the chemical symbol, the mass number above it and the atomic number below it, as in ¹⁴₆C.
Read the full explanation
Isotopes are atoms of the same element, so they share the atomic number, but they hold different numbers of neutrons and therefore have different mass numbers. Carbon-12 and carbon-14 both have six protons; one has six neutrons and the other eight. Since the electron arrangement is unchanged, isotopes of an element behave identically in chemical reactions but differ in mass and in nuclear stability, which is why carbon-14 is radioactive while carbon-12 is not. Examiners ignore any mention of electron numbers in the definition.
Your focus
- Describe what is meant by isotopes of an element in terms of protons and neutrons.
- Identify which of several nuclide symbols represent isotopes of the same element, and justify the choice.
- Calculate the number of neutrons in carbon-12 and carbon-14 and explain why both are still carbon.
Mass number, atomic number and isotopes exam tips
Marking Points
- one mark for isotopes being atoms of the same element, with the same number of protons
- one mark for stating that they have different numbers of neutrons
- one mark for the equivalent wording, the same atomic number but a different mass number
- one mark for reading the nuclide symbol correctly, mass number above and atomic number below
Examiner Tips
- 💡A two-mark definition needs both halves: same protons and different neutrons.
- 💡Naming an isotope as carbon-14 tells you the mass number, so the neutron count is 14 minus the atomic number.
- 💡Do not mention electrons in an isotopes definition; it wins nothing and can cost clarity.
Common Mistakes
- defining isotopes by a difference in the number of electrons, which the mark scheme ignores
- claiming isotopes of an element are different elements
- saying the atomic number differs between isotopes
- writing the mass number in the lower position of the nuclide symbol