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Atoms, Elements and Compounds

Isotopes

Cambridge IGCSE Chemistry

Isotopes

  • Isotopes are atoms of the same element with the same number of protons and different numbers of neutrons.
  • In two isotopes of an element, the number of protons and the number of electrons are the same; the number of neutrons is different.
  • An atom has no overall charge: its number of protons is the same as the number of electrons.

Symbols for atoms and ions

nucleon number 13 proton number 6 C
The nucleon number and the proton number written to the left of the symbol for carbon.

The charge on an ion is the difference between its protons and electrons: a 2− ion has two more electrons than protons.

Chemical properties of isotopes

Isotopes of an element have the same chemical properties because they have the same number of electrons, so the same electronic configuration.

Relative atomic mass

relative atomic mass = (relative mass of isotope × percentage abundance) for each isotope, added together100
Method: relative atomic mass from percentage abundances
  1. For each isotope, multiply its relative mass by its percentage abundance.
  2. Add these together for every isotope.
  3. Divide the total by 100, and write down the value in full.
  4. Round the final value.

Where the abundances are given as a ratio, divide by the total of the ratio parts instead of 100.

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