Every substance is not something solid, it consists of small particles that are molecules. Molecules from atoms. From this we can conclude that the determined mass of a substance can characterize the molecules and atoms of the constituent elements. At one time, Lomonosov devoted most of his work to this topic. However, many curious naturalists have always been interested in the question: "In what units is the mass of a molecule, the mass of an atom?"
But first, let's dive into history a bit
In the past, the mass of hydrogen (H) was always used in calculations per unit mass of an atom. And, proceeding from this, they made all the necessary calculations. However, most compounds are present in nature in the form of oxygen compounds, so the mass of an element's atom was calculated in relation to oxygen (O). Which is rather inconvenient, since it was necessary to constantly take into account the O:H ratio, equal to 16:1, in the calculations. In addition, studies have shown an inaccuracy in the ratio, it was actually 15.88:1 or 16:1.008. Such changes were the reason for recalculating the massatoms for many elements. It was decided to leave the mass value of 16 for O, and 1.008 for H. Further development of science led to the disclosure of the nature of oxygen itself. It turned out that the oxygen molecule has several isotopes with masses of 18, 16, 17. For physics, the use of a unit that has an average value is not acceptable. Thus, two scales of atomic weights were formed: in chemistry and physics. Only in 1961, scientists came to the conclusion that it was necessary to create a single scale, which is still used today under the name "carbon unit". As a result, the relative atomic mass of an element is the mass of the atom in carbon units.
Calculation methods
The mass of a molecule of any substance consists of the masses of the atoms that form this molecule. This implies the conclusion that the mass of a molecule must be expressed in carbon units, just like the mass of an atom, i.e. the relative atomic mass is determined taking into account the relative molecular mass. As you know, using Avogadro's Law, you can determine the number of atoms in a molecule. Knowing the number of atoms and the mass of the molecule, the atomic mass can be calculated. There are several other ways to define it. In 1858, Cannizzaro proposed a method by which the relative atomic mass is determined for those elements that are capable of forming gaseous compounds. However, metals do not have this ability. Therefore, to determine their atomic mass, a method was chosen that uses the dependence of the atomic mass and heat capacity of the correspondingsubstances. But all the considered methods give only approximate values of atomic masses.
How was the exact mass of an element's atoms calculated?
Scientific studies have shown that these approximate values can be used to determine the exact value. To do this, you only need to compare the given value with the equivalent. The equivalent of an element is equal to the ratio of the relative atomic mass of the element to its valency in the compound. From this ratio, the correct relative atomic mass of each element was determined.
