Atomic mass reflects the size of an atom. Technically, the atomic mass is the sum of the protons, neutrons, and electrons within an atom. However, electrons are so small that their masses are considered to not contribute to the mass of the atom. Thus, the number of protons and neutrons found in the nucleus of an atom is used to determine the atomic mass of an atom.
Therefore, the atomic mass of an element is close to the mass number of an element. The mass number of an atom is the number of protons and neutrons contained within the nucleus of an atom.
The relative atomic mass (sometimes called atomic weight) of an atom can be found by finding the average mass of all the atoms of an element. Relative atomic mass is calculated by taking the sum of the relative abundance of all the isotopes that occur naturally for that element. Isotopes are atoms of the same element that have different numbers of neutrons.