Define the term ‘isotope’.

The term 'isotope' refers to atoms of the same element that have identical numbers of protons but different numbers of neutrons. This variation in neutron count results in different mass numbers for these isotopes. For example, hydrogen has three isotopes: protium (with no neutrons), deuterium (with one neutron), and tritium (with two neutrons). Each isotope has distinct nuclear properties, such as stability and radioactive behavior, which can impact their applications in fields such as medicine, nuclear energy, and research.

The focus on differences in neutron count is crucial because it directly influences the physical and chemical properties of the isotopes, as well as their behavior in nuclear reactions. This is why the choice emphasizing the variation in neutron numbers is the most accurate depiction of what isotopes are.