Atoms with the same atomic number but different mass numbers are called isotopes.

Here's why:

* Atomic Number: This represents the number of protons in an atom's nucleus. It defines the element.

* Mass Number: This represents the total number of protons and neutrons in an atom's nucleus.

Since isotopes have the same atomic number, they are the same element. However, they have different numbers of neutrons, which results in different mass numbers.

Example:

Carbon-12 and Carbon-14 are both isotopes of carbon. They have the same number of protons (6), but Carbon-14 has 8 neutrons, while Carbon-12 has 6 neutrons. This makes their mass numbers different (12 and 14, respectively).