Two atoms that have the same atomic number but different mass numbers are called

two atoms that have the same atomic number but different mass number asre called isotopes

Explanation:

The atoms are identified by the atomic number which is the number of protons.

That means that all the atoms of an element have the same number of protons (and electrons).

For example all the atoms of hydrogen have 1 proton (atomic number 1), all the atoms of gold have 79 protons (atomic number 79).

On the other hand, the number of neutrons of an element may differ, and the name to differentiate the atoms with the same number of protons and different number of neutrons is isotope.

The mass number of an atom is the sum of the number of protons and number of neutrons.

So, the isotopes of the same element have different mass number.

Most elements in nature have isotopes. For example, hydrogen has three most stable isotopes: protium (1 proton, none neutron), deuterium (1 proton,1 neutron), and tritium (1 proton, 2 neutrons).