Atoms with the same atomic number but different mass numbers are called:

The correct answer is Isotopes.

An atom is identified by two numbers: its atomic number (Z), the number of protons in the nucleus, and its mass number (A), the total number of protons and neutrons. Atoms that share the same atomic number but have different mass numbers are called isotopes. They are atoms of the same element (same number of protons, hence same chemical identity) that differ only in their number of neutrons.

For example, the three naturally occurring isotopes of hydrogen — protium (1H), deuterium (2H) and tritium (3H) — all have atomic number 1 (one proton) but mass numbers 1, 2 and 3 because they have 0, 1 and 2 neutrons respectively. Similarly, carbon-12 and carbon-14 are isotopes of carbon (both Z = 6, but A = 12 and 14).

Distinguishing the related terms: Isobars have the same mass number but different atomic numbers (e.g. argon-40 and calcium-40). Isotones have the same number of neutrons but different atomic and mass numbers. Isomers (nuclear isomers) have the same atomic number and the same mass number but differ in nuclear energy state. Only isotopes match the definition of equal atomic number with unequal mass number.