From the school chemistry course, we know that if you take one mole of a substance, it will contain 6.02214084 (18) • 10 ^ 23 atoms or other structural elements (molecules, ions, etc.). For convenience, the number of Avogadro is usually written in the following form: 6.02 • 10 ^ 23.
However, why is the Avogadro constant (in Ukrainian, “Avogadro became”) equal to that value? There is no answer to this question in textbooks, and historians from chemistry offer a variety of versions. It seems that the number of Avogadro has some secret meaning. After all, there are magic numbers where some attribute the number “pi”, Fibonacci numbers, seven (eight in the east), 13, etc. We will fight with the information vacuum. We will not speak about who Amedeo Avogadro is, and why, in addition to the law formulated by him, the found constant was also named a
crater on the moon, we will not speak. A lot of articles have already been written about this.
To be precise, Amedeo Avogadro was not involved in counting molecules or atoms in any particular volume. The first to try to figure out how many gas molecules
contained in a given volume at the same pressure and temperature, was Joseph Loshmidt, and that was in 1865. As a result of his experiments, Loshmidt came to the conclusion that in one cubic centimeter of any gas under ordinary conditions there are 2.68675 • 10 ^ 19 molecules.
Subsequently, a large number of independent methods were invented for 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 are trying to further increase the accuracy of the assessment in order to introduce a new definition of the term “kilogram”. So far, the kilogram is compared with the selected 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 “quantity of substance”. The main unit for measuring quantity is mole. According to 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 some example. As is known from the definition of an atomic unit of mass, 1 amu corresponds to one twelfth of the mass of one carbon atom 12C and is 1.66053878 • 10 ^ (- 24) grams. If you multiply 1 amu to 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 beryllium atom is 9.01 amu Let's calculate what one mole of atoms of this element is equal to:
6.02 x 10 ^ 23 mol-1 * 1.66053878x10 ^ (- 24) gram * 9.01 = 9.01 gram / mol.
Thus, it turns out that the molar mass numerically coincides with the atomic one.
The Avogadro constant was specially chosen so that the molar mass corresponds to an atomic or dimensionless quantity — the relative molecular (atomic) mass. We can say that the Avogadro number is due to its appearance, on the one hand, to the atomic unit of mass, and on the other hand, to the generally accepted unit for comparing mass - a gram.