The idea of the atomic structure of matter remained a purely philosophical speculation until the beginning of the 19th century when the foundations of chemistry as a science were formed. Chemists were the first to discover that many substances decay into simpler components during the reaction process. For example, water decomposes into hydrogen and oxygen. However, some substances – the same hydrogen and oxygen – cannot be decomposed into components by chemical reactions. Such substances are called chemical elements. By the beginning of the XIX century, about 30 chemical elements were known (at the time of writing this article, over 110 were discovered, including artificially obtained under laboratory conditions, see the Periodic System). In addition, it was found that in the course of chemical reactions, the quantitative ratio of the substances participating in this reaction does not change. So,
The first meaningful interpretation of these facts was suggested by John Dalton, whose name is immortalized in the law of Dalton discovered by him. In his chemical experiments, he investigated the behavior of gases (see Boyle-Mariotte Law, Charles Law and the Basic Law of Thermodynamics), but his range of interests was not limited to this. In 1808, he began publishing his fundamental two-volume work, The New System of Chemical Philosophy, which radically influenced the further development of chemistry. In this work, Dalton suggested that it is only possible to comprehend and interpret the latest achievements of experimental chemistry by accepting that each chemical element corresponds to a unique atom in these experiments and that it is the mixing and combining in various proportions of these atoms that leads to the formation of naturally observable chemicals. For example, water, according to Dalton, consists of a combination of two hydrogen atoms and one oxygen atom (the well-known formula H2O). The fact that all atoms of one species are indistinguishable among themselves has successfully explained why in chemical reactions they are always found in unchanged proportions. So, in the case of water, two hydrogen atoms are always the same, wherever we take this water and are always in the same connection with a single oxygen atom.
For Dalton, as for Democritus, the atoms remained indivisible. In the drafts and books of Dalton, we find drawings where the atoms are represented in the form of balls. However, the main thesis of his work-that each chemical element corresponds to a particular type of atom-formed the basis of all modern chemistry. This fact remains unalterable even now when we know that each atom in itself is a complex structure (see Rutherford’s experiment) and consists of a heavy, positively charged nucleus and light, negatively charged electrons orbiting the nucleus. It is sufficient to turn to the complexities of quantum mechanics (see also Bohr Atom and the Schrodinger equation) in order to understand that the concept of the atom did not exhaust itself in the 21st century.