One of two or more species of atom having an identical atomic number but differing in mass number is called an isotope. Isotopes are variants of an element that have the same number of protons (same atomic number) but differ in the number of neutrons (different mass numbers).
For example, carbon has three naturally occurring isotopes: carbon-12, carbon-13, and carbon-14. These isotopes all have six protons, which is the atomic number of carbon, but they have different numbers of neutrons. Carbon-12 has 6 neutrons, carbon-13 has 7 neutrons, and carbon-14 has 8 neutrons.
Isotopes exhibit similar chemical behavior since they have the same number of protons and electrons, which determine the element's chemical properties. However, they may differ in their stability and nuclear properties due to the varying number of neutrons. Isotopes can have different half-lives, radioactivity, and nuclear reactions based on their neutron composition.