Atomic theory

The atomic theory is a Théorie on the nature of lat matter which stipulates that the matter is made up of units discretes called atoms which supplanted old the beliefs which the matter can be broken up into infinitely small divisions.

The modern atomic theory

Birth

At the beginning of the XIXe century, John Dalton developed his atomic theory in which he proposed that each element is composed of atoms of only one and single type and that these atoms are immutable and indestructibles but can combine of more complex structures. One does not know with the Juste how Dalton arrived at this conclusion, except that this theory allowed to him to explain several chemical phenomena that is to him its contemporaries studied at the time.

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

The second was the law of the defined proportions, proven by Joseph Louis Proust in 1799. This law says that if a chemical compound is broken up into its components, the masses of the components will have the same proportions, it does not matter the quantity or the source of the original substance. Proust synthesized copper carbonate by several methods and observed that in each case the ingredients combined in the same proportions as those which it obtained by breaking up natural copper carbonate.

Dalton studied work of Proust and developed its own extension, the law of the multiple proportions: if two elements form between them several compounds, the reports/ratios of the masses of the second element which combine with the fixed mass of the first element will be small integers. A pair of substances that Dalton is supposed to have studied is the nitric oxyde (NO) and the Oxygène in a gas state (O₂). In a combination, they formed N₂O₃, but when it doubled the quantity of oxygen (a 1:2 report/ratio), it obteint the nitrogen dioxide (NO₂).

4NO + O₂ → 2N₂O₃

4NO + 2O₂ → 4NO₂

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

In 1803 Dalton its list of atomic weights published for several substances. It with calculated these weights thanks to the relative masses with which they combined; by taking hydrogen like basic unit. However, Dalton did not understand that with certain elements, the atoms exist in molecule-by example, gas oxygen exists in O₂. He also believed that the simplest compound between two elements is always a molecule? of each (for example, he believed that the formula of water was HO and not H₂O). Moreover, its equipment was little-precis. Result, its list was defective. Like example, it considered oxygen as being 5.5 times more massive than hydrogen, whereas actually it is 16 times more massive than hydrogen.

The error of Dalton was corrected in 1811 by Amedeo Avogadro. Avogadro with proposed that two equal volumes of two different gases, at equal temperature and pressure, have equal numbers of particles (in other terms, the mass of the particles does not affect volume). This discovery him with license to deduce diatomic nature from many elements by studying volumes in which they combine?. Like example: if two liters of hydrogen combine with one liter of oxygen to form two liters of vapor water (at temperature and constant pressure), that means that the oxygen molecules are divided into two to form with hydrogen two water molecules. Avogadro thus provides more correct estimates for several elements.

In 1827, Robert Brown observed that the particles of dust floating in water gigotent without any apparent reason. In 1905, Albert Einstein proposed that this phenomenon is due to the water molecules which bombard dust unceasingly. It describes this gigotement using a mathematical model which was validated in 1908 by Jean Perrin. This result proved the existence of the atoms definitively.

The discovery of the subatomic particles

One thought that the atoms were the fundamental particles of the matter, up to 1897 when JJ Thomson découvra the electron while experminantant with the cathode rays. A Crookes tube one is tubed out of sealed glass containing two electrodes separated by a vacuum. When a potential eletric is applied between the two élécrtodes, of the cathode ray are produced. These rays produce a luminous point with the other end of the tube. One had oneself already that the cathode rays could be deviated by a magnetic field; Thomson will decouvra that they could be also deviated by a field elecrtric. Thomson thus concludes from it that these ray were composed of particles of negative charge.

Thomson croya that these particles came from the atoms of the élécrtode. If the atoms are composed of these particles, that want-to say that the atoms are divisible. In order to explain the neutral load of the atoms, Thomson proposed that its negative particles were distributed in a uniform cloud of load positive.

Simple: Atomic theory

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