The idea that all substances in the world are composed of atoms was born in the 5th century BC, when the ancient Greek thinker Democritus suggested that everything that exists consists of the smallest movable particles. Then there was no technical means to test the truth of this hypothesis. One could only assume that, most likely, it is not devoid of meaning. For many centuries after Democritus, atoms were periodically recalled, but this hypothesis was not popular in those distant times. In the 19th century, the atom hypothesis reappeared on the scientific horizon.
Model describing the picture of the world
Scientists were looking for a model that could satisfactorily describe the picture of the real world. The atom turned out to be a very suitable model. Although atoms cannot be seen, by admitting their presence, scientists could explain what they observed in their experiments and in nature.
Scientists were convinced of the existence of atoms long before they could prove it. The model worked, although no one could prove it to be true. For example, at the beginning of the 19th century, the English scientist John Dalton, studying the laws of the course of chemical reactions, discovered that two substances chemically react always in the same constant proportion. For example, combining one part of oxygen and two parts of hydrogen gives water.
This made it possible to assume that the atoms of one substance, equal in mass to each other, were combined with the atoms of another substance (that is, with atoms of a different mass). In the case of water formation, one oxygen atom combines with two hydrogen atoms. The atomic model helped to understand what exactly Dalton observed in his experiments. There is one more simple piece of evidence for the existence of atoms.
If you look through the microscope at the pollen particles suspended in water, you can see that they make chaotic movements. Why? Scientists have suggested that this may be due to the fact that pollen particles collide with numerous atoms or groups of atoms, which scientists have called molecules (for example, in water, particles collide with water molecules).
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What is an atom made of?
Scientists who agreed with the atomic theory believed that the atom consists of small electrically charged particles - positive and negative, which, when combined in an atom, as in a ball, as a result, neutralize each other and make the atom as a whole electrically neutral. But in 1907, the English physicist Ernest Rutherford proved with his experiments that this is not entirely true.
Rutherford bombarded gold foil with a high-speed beam of positively charged particles. He believed that the particles would go through the foil. Some of the positively charged particles did fly through the foil. And some couldn't do it. Moreover, they ricocheted into the experimenter, as if they were repelled by some force hidden in the foil. Rutherford was surprised. He said that it was like starting to burn a pot on fibrous paper and suddenly see this pot flying off the potter's forehead.
Atom nucleus
Rutherford's experiments helped to confirm the atomic hypothesis and to understand how the atom works. It became clear that positive and negative particles are not evenly located in the atom. If this were so, then the positively charged particles in Rutherford's experiment would not be repelled with such force. This means that the nucleus of an atom is not neutral. In the middle of the atom there is a dense tangle of particles, that is, in the middle, which is called the nucleus of the atom, there are positively charged protons and neutral neutrons.
At a considerable distance from the nucleus, negatively charged particles - electrons - orbit around it. Since the positive charge is equal in magnitude to the negative, the atom as a whole is electrically neutral. It carries no charge.
But the core itself is the concentration of a positive charge. Many of the positive particles in Rutherford's experiment flew too close to the positively charged nucleus of the gold atom. Since the positive charges are mutually repelled, these particles flew back towards the experimenter. This gave him an idea of how atoms actually work.
Quarks
Protons and neutrons are particles that, although small themselves, are made up of even smaller particles called quarks. Scientists today believe that protons and neutrons are made up of even smaller particles called quarks.
Quarks are a new model to better explain the behavior of atoms in the real world. And just as scientists used to seek experimental evidence for the existence of atoms, now they are looking for real confirmation of the existence of quarks.