‘The chemical brothers: why isotopes are useful’ explains the chemistry and application of isotopes. The arrangement of isotopes in the Periodic Table by weight did not always make sense, until Francis Aston discovered the presence of isotopes: chemically identical atoms which differ only in atomic weight due to differing numbers of neutrons. Radioactive isotopes differ in the stability of their nuclei. Measuring the speed of decay allows scientists to date archaeological finds, and even the universe itself. Stable isotopes can be used to give a record of climate change. Isotopes are also commonly used in medical imaging and cancer treatment.