What are TWO substances that have the same atomic number, but different atomic masses?
All isotopes fit your proposed criteria.
Two atoms with the same number of protons but different number of neutrons are called isotopes, examples are
Some ratios of protons to neutrons are more stable than other ratios. Carbon 12 is the most common form of Carbon. Carbon 12 is very stable. Carbon 14 is a very rare form of Carbon. Carbon 14 is radioactive, it breaks down into a beta particle and forms Nitrogen 14.
All forms of Uranium are radioactive, but some are more radioactive than others.
All atoms of the same element have the same number of protons (and electrons in non ionic forms) but can vary in the number of neutrons. These variations are called isotopes.