Atoms of the same element that have different numbers of neutrons are called isotopes. Isotopes have the same number of protons (which defines the element) but differ in their neutron count, resulting in different mass numbers. For example, carbon has isotopes such as carbon-12, carbon-13, and carbon-14, which all have 6 protons but 6, 7, and 8 neutrons, respectively. Isotopes generally share the same chemical properties because they have the same number of protons and electrons but may differ in physical properties like mass and stability.