Scientists needed a consistent way to count the unimaginably small particles that make up matter to understand chemical reactions. This necessity led to the development of a unit that links the mass of a substance to the number of its constituent particles. The answer to who “discovered” this unit, known as the mole, and the associated constant, Avogadro’s number, is not a simple one, as it involves the cumulative work of several minds across more than a century of scientific inquiry.
Defining the Chemical Unit
The mole (symbol: mol) is the official unit of measurement in the International System of Units (SI) for the amount of a substance. The mole is defined as the amount of substance that contains exactly 6.02214076 x 10^23 elementary entities, such as atoms, molecules, or ions. This fixed numerical value is known as the Avogadro constant (N_A). The concept works like a chemist’s “dozen,” allowing scientists to use measurable masses in grams to represent a specific, known count of particles. Because of this relationship, the molar mass of a substance—its mass in grams per mole—is numerically equal to its atomic or molecular mass.
Avogadro’s Foundational Hypothesis
The intellectual journey toward the mole began in 1811 with the Italian physicist Amedeo Avogadro. He proposed a revolutionary hypothesis concerning the nature of gases and their components. This hypothesis stated that equal volumes of different gases, when measured at the same temperature and pressure, contain an equal number of molecules.
Avogadro’s insight helped resolve a major conflict in early chemistry by clearly distinguishing between an atom, the smallest indivisible unit of an element, and a molecule, which is a compound or an agglomeration of atoms. He correctly deduced that common elemental gases like hydrogen and oxygen exist not as single atoms but as diatomic molecules (e.g., H2 and O2). This explained puzzling experimental results, such as the observation that two volumes of hydrogen react with one volume of oxygen to produce two volumes of water vapor.
Despite its profound implications, Avogadro’s work was largely ignored by the scientific community for nearly fifty years. The distinction between atoms and molecules remained unclear for decades, and his hypothesis only gained traction after it was championed by the Italian chemist Stanislao Cannizzaro at the Karlsruhe Congress in 1860. Avogadro’s contribution was purely theoretical; he never determined the actual numerical value of the number of particles that would later bear his name.
Calculating the Constant and Naming the Unit
The step from Avogadro’s theoretical hypothesis to a measurable, numerical constant involved the efforts of several 19th and 20th-century scientists. The first known estimate was made by the Austrian physicist Josef Loschmidt in 1865. Loschmidt did not directly calculate the Avogadro constant, but instead determined the number of gas molecules in a unit volume, a value now known as the Loschmidt constant (n_0).
Loschmidt used the kinetic theory of gases and estimated the size of air molecules to be roughly one nanometer, which was remarkably close to the correct order of magnitude for the time. By relating the average distance a molecule travels before colliding with another (the mean free path) to its size and the volume it occupies when liquefied, he derived a value that provided the first tangible numerical link between the macroscopic and microscopic worlds. Loschmidt’s calculation paved the way for determining the Avogadro constant, which is a simple conversion from his number using the molar volume of an ideal gas.
The term “mole” was introduced much later by the German physical chemist Wilhelm Ostwald. Ostwald coined the term Mol—later translated to mole—in 1893 to describe the gram-molecular weight of a substance, deriving it from the Latin word moles, meaning “mass” or “bulk.” This provided a standardized unit for chemists to use in their calculations, even as debates about the physical reality of atoms persisted.
The final, definitive measurements of the constant were performed by the French physicist Jean Perrin in the early 20th century. Perrin conducted multiple, independent experiments, most notably by observing the Brownian motion of tiny particles suspended in liquid. Using Albert Einstein’s theoretical work on Brownian motion, Perrin was able to calculate the Avogadro Constant in several different ways, including measuring the vertical distribution of particles in an emulsion, which behaved like a miniature atmosphere. The consistent values he obtained from these diverse methods offered compelling experimental evidence for the existence of atoms and molecules and provided a highly accurate numerical value for the constant. It was Perrin who proposed naming the constant “Avogadro’s number” in honor of the scientist who had first theorized the relationship between gas volume and particle count a century earlier.