What atoms with the same atomic number but a different atomic mass.?
There are atoms that have the same number of protons (atomic number) but have a different number of neutrons (resulting in different atomic mass). You call these isotopes.
For example, consider carbon-12, carbon-13, and carbon-14. They all have the same atomic number since they are all carbon. However they have different mass numbers (12, 13, and 14) because each atom has a different number of neutrons. All three of them are called isotopes of carbon.