Atomic mass is the mass of a single atom, usually taken as the weighted average of the masses of the element's naturally occurring isotopes. It is normally expressed in atomic mass units. Because most elements are mixtures of isotopes, the atomic mass is generally not a whole number.

原子质量是单个原子的质量,通常取该元素天然存在的各同位素质量的加权平均值。它一般以原子质量单位表示。由于大多数元素是同位素的混合物,原子质量通常不是整数。