The atomic mass, also known as the atomic mass number or atomic weight, represents the average mass of an atom of a specific element. It takes into account the masses of all the isotopes of that element and their relative abundance.
Atoms of the same element can have different numbers of neutrons, resulting in different isotopes. Isotopes have the same number of protons (and hence the same atomic number) but differ in their mass numbers. The atomic mass of an element is a weighted average of the masses of its isotopes, where the weighting is based on the abundance of each isotope in a naturally occurring sample of the element.
The atomic mass is usually expressed in atomic mass units (amu) or unified atomic mass units (u). One atomic mass unit is defined as 1/12th the mass of a carbon-12 atom. Since the atomic mass is relative to carbon-12, it does not have any units.
The atomic mass listed on the periodic table represents the average atomic mass of the naturally occurring isotopes of an element, taking into account their abundance. It is a useful value for determining stoichiometry and calculating molar masses in chemical calculations.