Final answer:
Atoms of the same element that contain the same number of neutrons but different numbers of protons are called isotopes. They have the same atomic number but different mass numbers.
Step-by-step explanation:
Atoms of the same element that contain the same number of protons, but different numbers of neutrons, are known as isotopes. Isotopes of any given element all contain the same number of protons, so they have the same atomic number. However, isotopes of a given element contain different numbers of neutrons, therefore, different isotopes have different mass numbers. Knowing the mass number and the atomic number of an atom allows you to determine the number of neutrons present in that atom by subtraction.