The atomic mass of an atom refers to the mass of a single atom, usually expressed in atomic mass units (amu) or unified atomic mass units (u). It represents the total mass of the protons, neutrons, and electrons in the atom.
The atomic mass is different from the masses of other atoms because atoms of the same element can have different numbers of neutrons, resulting in different isotopes. Isotopes are variants of an element that have the same number of protons but different numbers of neutrons. Since the number of neutrons can vary, the atomic mass of an element is an average value that takes into account the masses and abundances of its isotopes.
The atomic mass listed on the periodic table is a weighted average of the masses of all naturally occurring isotopes of the element, considering their abundance in nature. This average is calculated by multiplying the mass of each isotope by its abundance and summing up the results, as described earlier.
For example, the atomic mass of gold is approximately 196.9665 amu. Gold has multiple isotopes, but the two most abundant isotopes are gold-197 (with 79 protons and 118 neutrons) and gold-196 (with 79 protons and 117 neutrons). The atomic mass considers the masses and relative abundances of these isotopes, resulting in the average atomic mass of gold.