An isotope is a variant of a chemical element that has the same number of protons but a different number of neutrons in its nucleus. This difference in neutron count means isotopes of the same element can have different atomic masses and, in some cases, different physical or chemical properties. Some isotopes are stable, while others are radioactive and can emit harmful radiation. Radioactive isotopes, such as strontium-90, are of particular concern due to their potential health and environmental hazards.