An isotope is a variant of a chemical element that has the same number of protons but a different number of neutrons in its atomic nucleus. This means isotopes of a particular element have the same atomic number (which defines the element) but different mass numbers. Due to the varying number of neutrons, isotopes of an element may exhibit different atomic masses.
For example, carbon has three isotopes: carbon-12, carbon-13, and carbon-14. All three isotopes have six protons (defining them as carbon) but differ in the number of neutrons. Isotopes can have different physical properties, such as stability and radioactivity, but they share similar chemical properties because they have the same number of protons and, therefore, the same electron configuration.