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Atoms and molecules

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All substances consist of tiny particles - atoms. Atoms incorporate in the molecules largest from which have the difficult structure consisting of thousand of atoms.

About that all real consists of particles, ancient Greeks knew still. About 420 BC philosopher Demokrit has supported a hypothesis that the matter consists of tiny, indivisible particles. In a Greek way atomos means "эхфхышь№щ" therefore these particles named atoms.

Other philosophers adhered to other point of view, and in IV century BC Aristotle was expressed in support of opinion according to which the matter consists of various combinations of so-called four elements - the earths, air, fire and water. This idea was widely adopted and has laid down in an alchemy basis - the primitive form of the chemistry dominating in a science to a XVII-th century.

Creation "ІышъёшЁр цшчэш" was One of the alchemy main tasks; - a medicine which would allow the person to live eternally. Another consisted in creation of riches by transformation of usual metals into gold. Many alchemists asserted that have solved these problems, however anybody from them and has not achieved real success.

Revolution in a science

Some scientists continued to hold the opinion that the matter consists of atoms, but only in the XIX-th century beginning the experimental data confirming this theory have been received. The English chemist and writer John Dalton made experiments with gases and studied ways of their connection. So, he has found out that oxygen and hydrogen, forming water, always incorporate in the same proportions on weight. Other scientists also faced the similar data, but Dalton has realised for the first time their value. It has drawn a conclusion that substances consist of atoms, and that all atoms of simple substance have identical weight. At connection of simple substances of quantity of incorporating atoms are in a certain invariable proportion. Atomistika Daltona explained, why substances incorporate in an invariable mass proportion, and also was a basis for detailed studying of a matter. Substances consist of atoms and of what atoms consist? The first keys to a solution of this secret have appeared in the end of a XIX-th century when researchers studied electricity passage through the digit tubes containing rarefied air. Sometimes tube walls radiated green light at high voltage giving on two metal plates - electrodes. The luminescence arose at hit of invisible beams from a negative electrode, or the cathode, on tube walls.

In 1890th years the English physicist of J. Thomson has proved that these cathodic beams (as them then named) - not that other, as streams of negatively charged particles. It was supposed that these particles start with atoms though their arrangement in atoms remains not clear. Thomson has come out with the assumption that the atom can be similar to a Christmas pudding in which big, but easy on weight positively charged sphere is covered by the numerous negatively charged particles (elektronami). However various experiences on studying of a structure of atom have proved that it is certainly erroneous theory.

an atom Structure

an atom Structure In 1911 Ernest Rezerford, the British physicist, the native of New Zealand working together with Thomson, has offered the structure of atom really explaining it behaviour during experiments. Rezerford has assumed that the center (or a kernel) atom has a positive charge and rather big weight, and round a kernel the easiest rotate and negatively charged elektrony.

However Rezerford did not realise that usually in an atom kernel are both positively charged, and neutral particles. Existence of positively charged particles has been recognised in 1920, and they have received the name protons. In 1932 English physicist James Chedvik has opened not charged particles and named their neutrons. As a result the picture of a structure of atom has been finished and since then is a basis of our understanding of a matter.


Any substance in which all atoms have identical quantity of protons, is called as an element. Number of protons in each atom - atomic number of an element. There are 92 natural elements, their atoms have from 1 to 92 protons. Besides, some other elements with even great number of protons in atom can be received by means of the device under the name the accelerator of elementary particles. Iron, mercury and hydrogen concern natural elements.

In many substances atoms unite in the groups named molecules. So, gas hydrogen consists of molecules, each of which contains two atoms of hydrogen. Often, however, substance molecules consist of atoms more than one element. Such substances are called as connections. For example, water is connection where each molecule consists of two atoms of hydrogen and one atom of oxygen. In many molecules the much bigger quantity of atoms is. Some albuminous molecules represent difficult connections from several thousand atoms. Some natural elements meet only in connections. So, sodium is the metal so easily incorporating to other substances that he cannot be found out in the pure state. It is widely known in a combination to chlorine in the form of sodium chloride - table salt.


Communications Atoms in molecules communicate various ways, thus they divide among themselves elektrony or exchange them. Two simple kinds of a chemical bond are kovalentnaja and ionic.

Kovalentnaja communication arises, when atoms have the general elektrony. So, the molecule of hydrogen gas consists of two atoms of hydrogen connected kovalentnoj by communication. Unique elektron each atom of hydrogen rotates round kernels of both atoms, connecting them together.

In case of ionic communication one atom transfers elektrony to other atom. As a result there is the electric force connecting atoms together. As a rule, quantity of positively charged protons and negatively charged elektronov in atom equally. Their positive and negative charges counterbalance each other and consequently the atom has no general charge. However in the atom giving elektrony, surplus of a positive charge, and the atom receiving elektrony is created, gets the general negative charge. Such charged atoms are called as ions. Ions of opposite charges are drawn to each other, and this electric attraction keeps atoms together at ionic communication. For example, the table salt molecule is formed by means of ionic communication when the atom of sodium transfers elektron to atom of chlorine.

All atoms of one substance have identical quantity of protons, but various quantity of neutrons. So, the kernel of the majority of atoms contains six neutrons in carbon, but approximately in everyone 100-th of them is available seven neutrons. These various types of atoms of the same element are called as isotopes. All isotopes of the given element possess identical chemical properties - all of them incorporate to other substances and form the same chemical compounds. But separate physical properties of isotopes differ - for example, they have different freezing points or boilings.

Speaking about a concrete isotope of this or that element, scientists name its mass number. For example, carbon-12 is a usual natural isotope of carbon. Its atom contains six protons and six neutrons. More rare natural isotope, is in a kernel of which each atom a superfluous neutron, carbon-13 is called.

Nuclear weight

The Proton and a neutron have almost identical weight which more than in 1800 times exceeds weight elektrona. Therefore so far as concerns weight of atom, as a rule, an error will not refer to its mass number.

The Nuclear weight of an element, or its relative nuclear weight, usually represents average weight of a mix of the isotopes meeting in the nature. The molecular weight of substance, or its relative molecular weight, is the sum of nuclear scales of all atoms of one molecule of the given substance.

Multidifficult atom

Since then scientists experimenting accelerators have opened hundreds other kinds of particles in atoms. But, fortunately, the simple model of atom is sufficient to explain the most part of properties of a matter.