Isotopes are characterized by:

Isotopes are variants of a particular chemical element that share the same number of protons, which defines the atomic number, but differ in the number of neutrons, leading to different mass numbers. This fundamental aspect means that isotopes of an element have the same atomic number but different mass numbers, validating the first part of the correct answer choice.

Moreover, because isotopes of an element possess the same number of protons and electrons, they also display identical chemical properties. The arrangement of electrons influences how an element reacts chemically, and since isotopes maintain the same electron configuration, their chemical properties remain unchanged.

Therefore, the correct answer encompasses both the fact that isotopes share the same atomic number and have differing mass numbers while also reflecting that their chemical properties are identical. This dual recognition of both their atomic similarities and chemical behavior justifies the selection of this particular answer.