What distinguishes isotopes of an element?

Isotopes of an element are distinguished by the number of neutrons present in their atomic nuclei. While isotopes have the same number of protons and electrons, which define the element and its chemical behavior, the varying number of neutrons results in different atomic masses. This variation in neutron count leads to isotopes that can exhibit different stability and radioactive properties.

For example, hydrogen has three isotopes: protium, deuterium, and tritium. They all have one proton, but protium has no neutrons, deuterium has one neutron, and tritium has two. This difference in neutrons makes each isotope distinct in terms of mass and certain nuclear behaviors, while their chemical properties remain largely similar due to having the same number of protons and electrons.

Other factors, such as the number of protons or electrons, do not differentiate isotopes; rather, they define the element itself. Atomic mass reflects the average mass of all the isotopes of an element, but it is the neutron count that specifically characterizes each isotope.