阿伏伽德罗常数Avogadro constant

N_A 与其他常数关系密切：法拉第常数 F = N_A·e，摩尔气体常数 R = N_A·k_B（k_B 为玻尔兹曼常数）。它不是一个纯数，而是带单位 mol⁻¹ 的常数；去掉单位的数值常称为“阿伏伽德罗数”。

N_A ties together other constants: the Faraday constant F = N_A·e and the gas constant R = N_A·k_B, where k_B is the Boltzmann constant. It is not a pure number but carries the unit mol⁻¹; the bare numerical value is often called Avogadro's number.

常数以意大利科学家阿伏伽德罗命名，他在 1811 年提出“同温同压下相同体积的气体含有相同数目的分子”的假说，但他本人从未测定过这个数。1865 年洛施密特由气体分子运动论首次估算了单位体积气体中的分子数。1909 年佩兰通过布朗运动实验测定其值，并建议以阿伏伽德罗命名。

The constant honours the Italian scientist Avogadro, who proposed in 1811 that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, though he never measured the number. Loschmidt first estimated molecules per unit volume of gas from kinetic theory in 1865. In 1909 Perrin measured it through Brownian motion experiments and proposed naming it after Avogadro.

历史上曾用布朗运动、油滴实验（电子电荷与法拉第常数之比）、X 射线晶体学和放射性衰变计数等方法测定 N_A。21 世纪的国际阿伏伽德罗项目用近乎完美的 ²⁸Si 单晶球，测量其体积、晶格常数和质量，把相对不确定度降到 2 × 10⁻⁸ 量级，为 2019 年 SI 重新定义提供了依据。

N_A has been measured by Brownian motion, by the ratio of the Faraday constant to the electron charge from oil-drop experiments, by X-ray crystallography and by counting radioactive decays. In the twenty-first century the International Avogadro Project measured the volume, lattice spacing and mass of nearly perfect ²⁸Si spheres, reaching relative uncertainties of about 2 × 10⁻⁸ and supporting the 2019 SI redefinition.