Scientists did not immediately come to the correct understanding of the structure of the atom. The first atomic model was proposed by the English physicist J.J. Thomson, who discovered the electron. But his model came into conflict with the experiments of E. Rutherford to study the distribution of a positive charge in a microparticle. These Rutherford experiments played a major role in understanding how an atom works.
It was already known that the mass of an electron is thousands of times smaller than the mass of the particle itself. Rutherford made the assumption: since the atom as a whole is neutral, its bulk must be in the positively charged part. To confirm this hypothesis, Rutherford's experiments were reduced to the following.
He suggested using alpha particles to probe the atom. The mass of an electron is approximately 8000 times smaller than the mass of α-particles, and their speed is very high - it can reach twenty thousand kilometers per second. These were Rutherford's experiments on the scattering of alpha particles.
Atoms of heavy elements were bombarded with these particles. Due to the low mass, the electrons could not change the trajectory of α particles strongly. This could be done only by the part of the atom, positively charged. Therefore, by the nature of the scattering of alpha particles, it will be possible to find out the mass distribution inside the microparticle of the substance and the positive charge.
Rutherford's experiments had the following scheme. A radioactive substance was placed inside a lead cylinder. A narrow channel was drilled longitudinally in this cylinder. The flux of α particles from this channel fell onto a thin foil from the material under study (copper, gold, and others). Then, alpha particles were incident on a translucent screen that was coated with zinc sulfide. Each particle, colliding with the screen, gave a flash of light (scintillation), it could be seen through a microscope.
Rutherford's further experiments showed that a small number of alpha particles (approximately one out of two thousand) deviated by an angle of more than 90 °. This fact greatly puzzled Rutherford. He said that it was just as incredible as firing a shell at a piece of tissue paper and he would return to you and strike. Indeed, it is impossible to predict such a result based on the Thomson model, and Rutherford suggested that an α-particle can be thrown back only when the bulk of the atom is in a very small space. So Rutherford's experiments helped him come to a model of the nucleus. This body is small in size, where almost all the positive charge and the entire mass of the microparticle are concentrated.
The atomic model directly follows from the experiments that Rutherford conducted. The structure of the atom according to Rutherford's concept is as follows. A positively charged core is in the center. Since the atom is neutral, the number of electrons is equal to the ordinal number of the element in the Mendeleev periodic system. They move in a circle above the core, as the planets revolve around the Sun in their orbits. The motion of electrons is due to Coulomb forces. A hydrogen atom contains only one electron that revolves around its nucleus. Its atomic nucleus carries a positive charge and mass, approximately 1836 times the mass of an electron.
Such an atom model had experimental justification, but the stability of its existence cannot be explained on the basis of this model.
Electrons moving in an orbit must, according to the laws of classical mechanics, approach the nucleus due to energy losses and, in the end, fall on it. In fact, the electron does not fall on the nucleus. Microparticles of chemical elements are very stable and can exist for a very long time. The conclusion about the inevitable destruction of the atom due to energy losses, which is not consistent with Rutherford's experiments, is the result of applying the laws of classical mechanics to microscale phenomena. Consequently, the laws of classical physics are not applicable to the phenomena of the microworld.