The atomic weight of an element is the weighted average of the masses of its naturally occurring isotopes, each weighted by its natural abundance; this is the value shown for each element in the periodic table. Because most elements occur as a mixture of isotopes, the atomic weight is generally not a whole number. It is the quantity used in chemical calculations to relate the mass of an element to the amount of substance.

元素的原子量是其天然存在的各同位素质量按各自天然丰度加权的平均值;这正是周期表中每种元素所列的数值。由于大多数元素以多种同位素的混合物形式存在,原子量通常不是整数。它是化学计算中用来将元素质量与物质的量相联系的量。