Isotopes are versions of an element with different numbers of neutrons. Hydrogen has three naturally occurring isotopes. , sometimes called protium, contains one electron, one proton, and no neutrons. , called deuterium, contains one electron, one proton, and one neutron. , called tritium, contains one electron, one proton, and two neutrons. Hydrogen has no such isotope that contains three neutrons.

Each element is defined by the number of protons its atoms contain. For example, hydrogen has one proton, helium has two protons, and lithium has three protons. Each element also has a characteristic number of neutrons. For example, hydrogen has zero neutrons, helium has two neutrons, and lithium has four neutrons.

Some elements, however, also have different "versions" of themselves: atoms which have a different number of neutrons, called isotopes. For example, there are three isotopes of hydrogen. has one proton and zero neutrons. has one proton and one neutron. Lastly, has one proton and two neutrons. Carbon is another such element that has different isotopes.

By definition, isotopes of a given element have the same number of protons and electrons, but differ in the number of neutrons. This causes a difference in the mass number (protons + neutrons) as well. Neither the number of protons nor the number of electrons changes with different isotopes of the same element.

In order to find the molar mass of an atom from its isotopes and their natural abundances, use the following equation:

for all the given isotopes.

Since chromium has four isotopes, we will write the following equation to find its atomic mass: