相对原子质量Relative atomic mass

Ar = Σ(相对同位素质量 × 丰度) ÷ 100。例如硼含 19.9% ¹⁰B 和 80.1% ¹¹B，Ar = (10 × 19.9 + 11 × 80.1) ÷ 100 = 10.8。同位素的质量和丰度可由质谱仪测得：质谱图上每个峰的位置给出 m/z，峰高给出相对丰度。

Ar = Σ(relative isotopic mass × abundance) ÷ 100. Boron, 19.9% ¹⁰B and 80.1% ¹¹B, gives Ar = (10 × 19.9 + 11 × 80.1) ÷ 100 = 10.8. Isotopic masses and abundances come from a mass spectrometer: each peak's position gives m/z and its height the relative abundance.

相对分子质量 Mr 等于分子中各原子相对原子质量之和，如 Mr(H₂O) = 2 × 1.0 + 16.0 = 18.0；离子化合物常称相对式量。摩尔质量以 g mol⁻¹ 为单位时，数值与相对原子质量或相对分子质量相等。

Relative molecular mass, Mr, is the sum of the relative atomic masses of the atoms in a molecule: Mr(H₂O) = 2 × 1.0 + 16.0 = 18.0; for ionic compounds the term relative formula mass is used. Molar mass in g mol⁻¹ has the same numerical value as Ar or Mr.

道尔顿最早以氢为 1 编制原子量表，后来化学界改以氧为 16 作标准。由于物理学家用 ¹⁶O、化学家用天然氧，两套数值略有差别。1960–1961 年 IUPAP 和 IUPAC 统一采用 ¹²C = 12 作为基准。因天然同位素组成存在差异，IUPAC 对部分元素（如 H、C、Cl）给出原子量的取值区间。

Dalton's first tables set hydrogen at 1; chemists later used oxygen = 16. Because physicists used ¹⁶O and chemists natural oxygen, two slightly different scales arose. In 1960–1961 IUPAP and IUPAC agreed on ¹²C = 12 as the common standard. Because natural isotopic composition varies, IUPAC now gives ranges for the atomic weights of some elements such as H, C and Cl.