By the end of the 19th century, it became clear from the development of chemistry that substances are composed of atoms. Guesses about their existence arose earlier, in ancient Greek science, from the desire to give nature a coherent mathematical explanation. By 1804, John Dalton formulated the law of multiple proportions in chemical reactions: if two elements can form a compound, then the ratio of masses of these elements is a small whole numbers. This could be explained by the fact that each substance consists of identical particles that simply combine with particles of other substances in chemical reactions. Then the masses of these particles will determine the mass proportions of the reacting substances. As a result of Avogadro’s experiments with gases, it turned out that instead atoms, molecules were hidden behind these particles. As a confirmation, using a microscope in 1827, Robert Brown discovered a disorderly movement of dust particles under the impact of molecules in water. Then they started to indicate atomic masses of substances in units of the mass of the hydrogen atom. It became clear that substances exhibit similar properties at equal intervals of atomic mass when they are arranged according to the increase in atomic mass. Thus the periodic table of substances was formed by 1869, and by the beginning of the next century it became known that the ordinal number of its elements simply shows the number of electrons in an atom.
Please take care of the atomic structure 🙂