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

isotopes

Explanation:

The atomic number is the number of protons in the atom.

Each element is identified by the atomic number. That means that all the atomos of a giiven element have the same number of protons.

On the other hand, the atoms of a same element may differ in the number of neutrons.

The neutrons are as massive as the protons, so the mass numbers are the sum of the number of protons and number of neutrons.

So, atoms of the same element but different number of neutrons have different mass numbers are named isotopes.

Most elements in nature have isotopes. For example, there are three atoms of potassium in the nature whose mass numbers are:

1) Isotope K-39: mass number 39; it has 19 protons and 20 neutrons
2) Isotope K -40: mass number 40; it has 19 protons and 21 neutrons
3) Isotope K-41: mass number 41; it has 19 protons and 22 netrons.