Atoms with the same mass number but different atomic numbers are called isobars. The mass number (A) is the sum of protons and neutrons. Atomic number (Z) is the number of protons. Therefore, isobars have different numbers of protons but the same total number of nucleons.

Example: ${\mathrm{Ar}}_{18}^{40}$ and ${\mathrm{K}}_{19}^{40}$ both have A=40, but Z=18 and Z=19 respectively.

Final Answer: isobars