An isotope is an atom that contains a different number of neutrons than its base chemical element. Each element is defined by its respective number of protons. The atomic number of an element refers to its proton count and establishes that particular element within the periodic table.
In non-ionized elements, the number of protons is equal to the number of electrons, which establishes a neutral charge. While electrons have mass, it is negligible when added to the atomic mass of an element. Atomic mass, therefore, refers to the total number of neutrons and protons. Isotopes, like base elements, are designed by their atomic mass, as in the example of hydrogen-3.