AQA GCSE Physics

Revision Notes

4.1.4 Isotopes


  • Although the number of protons in a particular element is always the same, the number of neutrons can be different
  • Isotopes are atoms of the same element that have an equal number of protons but a different number of neutrons
  • In the diagram below are three isotopes of Hydrogen:

Isotopes of Hydrogen, downloadable AS & A Level Physics revision notes

Hydrogen has three isotopes, each with a different number of neutrons

  • Isotopes occur naturally, but some are more rare than others
  • For example, about 2 in every 10,000 Hydrogen atoms is Deuterium
    • Tritium is even more rare (about 1 in every billion billion hydrogen atoms)

Exam Tip

This topic is also covered in Chemistry, although some of the terminologies may be a little different. However, in Physics you must refer to neutrons when explaining isotopes.

Differences Between Isotopes

  • The number of neutrons in an atom does not affect the chemical properties of an atom, such as its charge, but only its mass
    • This is because neutrons have no charge but do have mass
  • In the periodic table, the mass number of Chlorine is often given as 35.5

Chlorine isotope, downloadable IGCSE & GCSE Physics revision notes

This section of a periodic table shows Chlorine as having a mass number of 35.5, but other elements have an integer mass number

  • The mass number of Chlorine is given as 35.5 because it has roughly equal numbers of isotopes with a mass number of 35, and of 36
  • The number of electrons and protons in different isotopes remains the same
  • Isotopes tend to be more unstable due to the imbalance of protons and neutrons

Worked Example

State the number of protons, neutrons and electrons in Chlorine-35 and Chlorine-36 atoms.

Step 1: Determine the number of protons

    • The atomic number is the number of protons
    • Both Chlorine-35 and Chlorine-36 have 17 protons

Step 2: Determine the number of neutrons

    • The mass number is the number of protons and neutrons
    • Chlorine-35 neutrons: 35 – 17 = 18 neutrons
    • Chlorine-36 neutrons: 36 – 17 = 19 neutrons

Step 3: Determine the number of electrons

    • The number of electrons is equal to the number of protons
    • Both Chlorine-35 and Chlorine-36 have 17 electrons

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