Atoms are made up of one or more small mass particles called "Nucleons". However, the weight of an atom is not just a simple multiple number equal to the number of nucleons, but rather a number related to the number of nucleons plus a second quantity (or amount) related to the numerical value of the amount of free energy that the free neutron kept after it dropped into the energy sink made by the gradual build up of the atom's other nucleons. And, since we are using the value of 12 for the mass of an atom of 6C12 (Carbon), and since that number also has an amount of remaining free energy, The mass number of the atoms of the other elements varies around the nucleon number and can be more or less than that value.
Atomic mass numbers are important values used to measure the relative stability and capability of atomic change of atoms in nuclear physics considerations. The CRC Handbook of Chemistry and Physics lists these numbers as do other publications. The mass number is also the amount of protons and neutrons added together.
Simply, mass number is the sum of the number of proton and the number of neutron.