The atomic theory was born as an opposite theory to the theory of continuous. The ancient Greek in the V Century B.C., created the first ideas. For them, atom meant indivisible; that is, the smallest part, uncuttable, from where all things are made. In our days atom, in Greek fashion, is synonym of elementary particle.
In spite of that Greek got their hypothesis from the sensorial evidence, it seems that they never tested the consequences of their theory. The experimental method will be invented 22 centuries later.
The experimental evidence and irrefutable proof of the existence of the atoms came centuries later. The experimental method, as we know in these days, was born in the XVII century. Therefore, 23 centuries passed before the physicists were able to get the first experimental evidences on the existence of the atoms. Those were established close to XIX century. The chemists did the pioneer works.
A. L. Lavoisier was a notable precursor in the XVIII century. John Dalton continued and crystallized his achievements. By himself Dalton was a pioneer. He did the most important contributions. He created the basis of modern chemistry. He decomposed and weighed hundreds of substances. He searched the proportions in which those substances combine. He enunciated the law of proportions.
The law of proportions required, by necessity, the hypothesis of the atoms. His investigations clarified that the atoms have a finite size and finite weight; that there are many classes of atoms; that the hydrogen is the lightest; that there are substances composed by two classes of atoms, combined one to one -like in the table salt-; or some combine two to one -like in the ordinary water-; and that other substances are formed by more than two different sort of atoms -like the sugar-. He detected simple substances. And obtained others by decomposition. These are formed by one class of atom. The central problem was to say when a substance could not be decomposed into others. Was it really elementary or was he unable so far to find the method to decompose it? Only the experience could indicate it.
The knowledge of Dalton let Mendeleev classify, by weight, the simple substances. He left empty spots, when he had not the element to fill them. The empty spots indicated necessarily the existence of some element not yet discovered. The helium is an example. The physicists firstly detected it in the sun, using the spectroscopy technique. And then in the earth. The Mendeleev table let foretell other elements. Those later were discovered. The classification scheme of Mendeleev also let formulate other problems that remained as mysteries up to the invention of the quantum mechanics.
Actually the physicists know 118 classes of atoms.
The one hundred and eighteen classes of different atoms is very far away from the ancient Greeks proposed simplicity. They supposed the existence of a few. Of different sorts as they called them. Besides, the atom of the chemists revealed itself as a too complex physical system, very far away from Democritus idea of atom. The atom of the chemists is a universe by itself.
That universe began to be unveiled at the end of the XIX century. In 1897 J.J. Thomson discovered the electron -an object much less light than the atom of hydrogen-. The electron is close to two thousands more light than the proton. The experimental evidences indicated that the electron came from inside the atom. Based on this discovery the physicists formulated the first model of protons and neutrons, without well defined position; the nucleus is surrounded by clouds of electrons without well defined position and momentum; and the electrons remain in well defined states of energy.
The chemists atom is a truly universe. Each one of its components has many physical properties, besides it mass. The spin is one of these. And additionally, as we will see in the next sections, the neutrons and protons from the nucleus resulted composed systems, integrated by many sort of particles.