From the school chemistry course we know that if we take one mole of any substance, then it will contain 6.02214084(18)•10^23 atoms or other structural elements (molecules, ions, etc.). For convenience, the Avogadro number is usually written in this form: 6.02 • 10^23.
However, why is the Avogadro constant (in Ukrainian “became Avogadro”) equal to this value? There is no answer to this question in textbooks, and chemistry historians offer a variety of versions. It seems that Avogadro's number has some secret meaning. After all, there are magic numbers, where some refer to the number "pi", fibonacci numbers, seven (eight in the east), 13, etc. We will fight the information vacuum. We will not talk about who Amedeo Avogadro is, and why, in addition to the law he formulated, the constant found, a crater on the Moon was also named after this scientist. Many articles have already been written about this.
To be precise, Amedeo Avogadro did not count molecules or atoms in any particular volume. The first to try to figure out how many molecules of a gas
contained in a given volume at the same pressure and temperature, was Josef Loschmidt, and it was in 1865. As a result of his experiments, Loschmidt came to the conclusion that in one cubic centimeter of any gas under normal conditions there are 2.68675 • 10^19 molecules.
Subsequently, a large number of independent ways were invented on how to determine the Avogadro number, and since the results for the most part coincided, this once again spoke in favor of the actual existence of molecules. At the moment, the number of methods has exceeded 60, but in recent years, scientists have been trying to further improve the accuracy of the estimate in order to introduce a new definition of the term “kilogram”. So far, the kilogram is compared with the chosen material standard without any fundamental definition.
But back to our question - why is this constant equal to 6.022 • 10^23?
In chemistry, in 1973, for convenience in calculations, it was proposed to introduce such a concept as "amount of substance". The basic unit for measuring quantity was the mole. According to the IUPAC recommendations, the amount of any substance is proportional to the number of its specific elementary particles. The proportionality coefficient does not depend on the type of substance, and the Avogadro number is its reciprocal.
For clarity, let's take an example. As is known from the definition of the atomic mass unit, 1 a.m.u. corresponds to one twelfth of the mass of one carbon atom 12C and is 1.66053878•10^(−24) grams. If we multiply 1a.u.m. by the Avogadro constant, you get 1.000 g/mol. Now let's take some chemical element, say, beryllium. According to the table, the mass of one atom of beryllium is 9.01 amu. Let's calculate what one mole of atoms of this element is equal to:
6.02 x 10^23 mol-11.66053878x10^(−24) gram9.01=9.01 gram/mol.
Thus, it turns out that the molar mass is numerically the same as the atomic mass.
Avogadro's constant was specifically chosen so that the molar mass corresponds to an atomic or dimensionless value - the relative molecular (atomic) mass. We can say that the Avogadro number owes its appearance, on the one hand, to the atomic mass unit, and on the other hand, to the generally accepted unit for comparing mass - the gram.
