How is the mass number of an element similar to an isotopes of the element?

it is Atomic mass.

I think it's atomic mass

the mass number is number of protons in an atom, plus the number of neutrons.

Mass number is the sum of the protons and neutrons in the nucleus of the atoms of an isotope.

Deuterium, an isotope of hydrogen, has an atomic mass of 2 since it has a neutron as well as a proton in the nucleas. Hydrogen oxide, H20 is known as water - deuterium oxide is known as heavy water.