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The atomic nature of matter (Atomic theory)

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The atomic theory is a theory of the nature of matter, that matter is composed of discrete units called atoms that has supplanted ancient beliefs that matter can be broken down into infinitely small divisions.

The ancient atomist theory

The philosophers of antiquity created the atomist theory. The main creators are: in India, Kanada, author of the Vaisheshika Sutra, in Greece, Leucippus, Democritus and Epicurus, and finally in Rome, Lucretius author of De natura rerum.

The modern atomic theory


In the early nineteenth century, John Dalton developed his atomic theory in which he proposed that each element is composed of atoms of one and only one type and that these atoms are immutable and indestructible but can be combined in more complex structures. Historians do not know how Dalton reached this conclusion, except that this theory allowed him to explain several chemical phenomena that he and his contemporaries were studying at the time.

The first was the law of conservation of the mass, formulated by Antoine Lavoisier in 1789, which says that the total mass of a chemical reaction remains constant (in other words, the reagents have the same mass as the products). From this, Dalton concludes that matter is fundamentally indestructible.

The second was the law of defined proportions, proved by Joseph Louis Proust in 1799. This law says that if a chemical compound is decomposed into its constituents, the masses of the constituents will have the same proportions, regardless of the quantity or source of the original substance. Proust synthesized copper (II) carbonate by several methods and observed that in each case the ingredients were combined in the same proportions as those obtained by decomposing the natural copper carbonate.

Proust studied tin oxides and found that their masses consisted of either 88.1% tin and 11.9% oxygen, or 78.7% tin and 21.3% oxygen (tin oxide II and tin dioxide). Dalton noted from its percentages that 100 g of tin will react with 13.5 g or 27 g of oxygen – 13.5 and 27 will form a ratio of 1: 2. Dalton is also believed to have studied nitrogen monoxide (NO) and oxygen in the gaseous state (O2). In one combination, they formed N2O3, but when he doubled the amount of oxygen (a ratio of 1: 2), he obtained nitrogen dioxide (NO2).

4NO + O2 → 2N2O3

4NO + 2O2 → 4NO2

These ratios were always small whole numbers – the law of multiple proportions. Dalton found that an atomic theory of nature explained this phenomenon well.

Dalton also used his theory to explain why water absorbs different gases in different proportions (for example, water absorbs carbon dioxide better than nitrogen). He proposed that this happens because of the different masses and sizes of the particles of these gases (the molecules of carbon dioxide are heavier and larger than those of nitrogen gas).

John Dalton's New System of Chemical Philosophy (Various atoms and molecules as depicted in John Dalton’s New System of Chemical Philosophy (1808).)

In 1803, Dalton published his list of atomic masses for several substances. He calculated these masses by the relative masses with which they combined, taking hydrogen as the basic unit. However, Dalton did not understand that with certain elements, atoms exist in molecules. For example, gaseous oxygen is actually oxygen O2 and not O. He also believed that the simplest compound formed by the reaction of two elements contains only one atom of each species (for example, he believed that the water formula was HO and not H2O). Moreover, his equipment was not very precise. His list was therefore wrong. Indeed, it has, for example, estimated the oxygen as being 5.5 times more massive than the hydrogen, whereas it is 16 times more massive than this one.

Dalton’s error was corrected in 1811 by Amedeo Avogadro. Avogadro proposed that two equal volumes of two different gases, at equal temperature and pressure, have equal numbers of particles (in other words, the mass of the particles does not affect the volume). This discovery allowed him to deduce the diatomic nature of many elements by studying the proportions in which these gases are combined with other elements. For example: if two liters of hydrogen combine with one liter of oxygen to form two liters of water in steam (at constant temperature and pressure), it means that the oxygen molecules divide in two to form with oxygen. hydrogen two molecules of water. Avogadro therefore provides more correct estimates for several elements. Because of this error, the atomic school was challenged by the equivalence school (equivalents based on combination weight ratios, volume ratios or substitution equivalents) of Marcellin Berthelot and William Hyde Wollaston, up to the publication in 1913 of Jean Perrin, Les Atomes.

In 1815, William Prout shows that the masses of atoms of known elements are proportional to the mass of a standard volume of these elements in gaseous form, he finds that the measured masses of known atoms is a multiple of the mass of that of hydrogen, and concludes that “hydrogen is the primordial substance, of which all atoms would be formed”. This hypothesis subsequently loses ground when more precise measurements show that this is not the case.

In 1827, Robert Brown observed that pollen particles floating in the water rattled for no apparent reason. In 1905 Albert Einstein showed that this phenomenon, called Brownian motion, was due to the molecules of water that constantly hit these particles. His mathematical model was validated in 1908 by Jean Perrin. This result proved directly the existence of molecules and atoms.

The discovery of subatomic particles

When Dalton’s model was accepted, scientists believed that atoms were the fundamental particles of matter until 1897, when J. J. Thomson discovered the electron by conducting experiments with cathode rays. A Crookes tube is a sealed glass tube containing two electrodes separated by a vacuum. When an electric potential is applied between the two electrodes, cathode rays are produced. These rays produce a bright spot at the other end of the tube. Scientists of that time knew that cathode rays could be deflected by a magnetic field; Thomson discovered that they could also be deflected by an electric field. He concluded that these rays were composed of negatively charged particles.

Thomson thought that these particles came from the atoms of the electrode. If atoms are composed of these particles, it means that atoms are divisible. In order to explain the neutral charge of atoms, Thomson proposed that these negative particles were distributed in a uniform cloud of positive charge.

In 1911, Charles Barkla shows that in the case of light atoms, an atom contains approximately half as many electrons as its atomic weight expressed as a multiple of the mass of hydrogen, with the exception of hydrogen itself.

Later, it was discovered the notion of neutrons and protons and elementary particles. This was the beginning of particle physics.

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