Final answer:
Isotopes of the same element differ in the number of neutrons in their atomic nuclei, resulting in different mass numbers but not altering the chemical properties.
Step-by-step explanation:
Isotopes of the same element have different numbers of neutrons. An atom's identity is defined by its atomic number, which corresponds to the number of protons in its nucleus. All isotopes of an element share the same number of protons and, therefore, the same atomic number; however, they differ in their mass number, which is the sum of protons and neutrons. Thus, it is the neutron count that varies among isotopes of a single element, leading to differences in atomic mass but not in chemical properties. For example, carbon-12 and carbon-14 are both isotopes of carbon with mass numbers of 12 and 14, respectively, because carbon-12 has six neutrons while carbon-14 has eight neutrons, but both have six protons.