The structure of matter has been studied since the fifth century BC, when the first idea about its constitution arose. The philosophers Leucippus and Democritus argued that matter could not be infinitely divided into an indivisible unity called an atom. These speculations were replaced by models based on experimental studies after thousands of years.
Based on the weighted laws of Lavoisier and Proust, scientist John Dalton, around the year 1808, elaborated his theory on matter, known as Dalton's atomic theory. The main conclusions of Dalton's atomic model were: matter is made up of extremely small particles called atoms; atoms are massive and indivisible spheres; atoms with the same properties constitute a chemical element; Different elements are constituted by atoms with different properties and chemical reactions are rearrangements, joining and separation of atoms.
After Dalton, based on experiments with electric charges, the English scientist Joseph John Thomson in the late nineteenth century concluded that the atom was not an indivisible sphere, as Dalton suggested. The experience that led to the elaboration of this model consisted in the emission of cathode rays, where negative particles were attracted by the positive pole of an external electric field. These negative particles were called electrons, and to explain the neutrality of matter, Thomson proposed that the atom be a sphere of positive electric charge, where the electrons would be evenly distributed, setting an electrical equilibrium.
Years later, in the early twentieth century, scientist Ernest Rutherford, using radioactivity, discovered that the atom was not a massive sphere, as Dalton's atomic theory suggested. Thus emerged a new atomic model.
Rutherford bombarded a 0.00001 cm gold sheet surrounded by a zinc sulfide screen, with α particles from the polonium element protected by a perforated lead block. This experience revealed that the vast majority of particles passed through the gold blade, while other particles passed and suffered slight deviations, and a very small amount did not cross the blade. The pathway followed by the α particles was detected due to the luminosity reflected on the zinc sulfide screen.
Comparing the number of particles emitted with those of deviated particles, Rutherford deduced that the mass of the gold blade would be located in small points, called nuclei, and that the radius of the atom should be 10,000 to 100,000 times greater than the radius of the nucleus. atom formed by empty spaces. Most particles have crossed the blade through these spaces. The explanation for the deviated α particles was given that the positive core of the gold slide repels the also positive alpha particles. The particles that did not cross would have collided head-on with these nuclei, being bounced.
Rutherford's atomic model concluded that the atom was composed of a small positively charged nucleus neutralized by a negative region, called the electrosphere, where electrons rotated around the nucleus.