What are the atoms and molecules. The structure of matter - molecules

Quantum   (from him. Quant- “Quantum” from lat. quantum   - “how many”) is an indivisible portion of some elementary particle or magnitude in physics (for example, the amount (portion) of electromagnetic radiation, which in a single act can emit or absorb or other quantum system; elementary particle, the same as a photon). The concept is based on the quantum mechanics idea that some physical quantities can take only certain values ​​(they say that the physical quantity quantized).

In some important particular cases, this value or step of its change can only be integral multiples of some fundamental value - and the latter is called quantum. For example, the energy of a monochromatic electromagnetic radiation of the angular frequency ω can take on the values ​​(N + 1/2) ℏω, where План is the reduced Planck constant and N is an integer. In this case, ℏω has the meaning of the energy of a quantum of radiation (in other words, a photon), and N is the meaning of the number of these quanta (photons). It is in this sense that the term quantum was first introduced by Max Planck in his classic work of 1900 - the first work on quantum theory that laid its foundation.

Since the beginning of the 1900s, a completely new physical concept has developed around the idea of ​​quantization, usually called quantum physics (for example, the amount (portion) of electromagnetic radiation that can emit or absorb in another single act or other quantum system; an elementary particle, the same photon).

Today, the adjective “quantum” is used in the title of a number of fields of physics (quantum mechanics, quantum field theory, quantum optics, etc.). The term quantization is widely used, meaning the construction of a quantum theory of a certain system or a transition from its classical description to a quantum one. The same term is used to denote a situation in which a physical quantity can take only discrete values ​​— for example, it is said that the energy of an electron in an atom is “quantized.” The term "quantum" itself currently has a rather limited use in physics. Sometimes it is used to designate particles or quasiparticles corresponding to the boson interaction fields (photon - electromagnetic field quantum, phonon - quantum field of sound waves in a crystal, graviton - a hypothetical quantum of the gravitational field, etc.), they are also referred to as “ excitation photons "or simply" excitations "of the corresponding fields.

In addition, according to tradition, the "quantum of action" is sometimes called Planck's constant. In the modern sense, this name may have the meaning that the Planck constant is a natural quantum unit of measurement of action and other physical quantities of the same dimension (for example, angular momentum).

Quark   - the fundamental particle in the Standard model, having an electric charge multiple e/ 3, and not observed in the free state, but part of the hadrons (strongly interacting particles, such as protons and neutrons). Quarks are unstructured, point particles; this is verified up to a scale of about 5 · 10 −18 m, which is about 20 thousand times smaller than the size of a proton.

Currently, there are 6 different "varieties" (more often they say - "aromas") of quarks, the properties of which are given in the table. In addition, for the gauge description of a strong interaction, it is postulated that quarks also have an additional intrinsic characteristic called “color”. Each quark corresponds to an antiquark with opposite quantum numbers.

The hypothesis that hadrons are constructed from specific subunits was first put forward by M. Gell-Mann and, independently of him, by J. Zweig in 1964.

Nuclones   (from lat. nucleus   - nucleus) - particles from which atomic nuclei are built. Nucleons are represented by protons and neutrons.

From the point of view of electromagnetic interaction, proton and neutron are different particles, since the proton is electrically charged, but the neutron is not. However, from the point of view of the strong interaction, which is decisive on the scale of atomic nuclei, these particles are indistinguishable, therefore the term “nucleon” was introduced, and the proton and neutron began to be considered as two different nucleon states differing by the projection of the isotopic spin. The proximity of the properties of the isospin states of a nucleon is one of the manifestations of isotopic invariance.

An atom is the smallest particle of a chemical element that retains all its chemical properties. An atom consists of a nucleus with a positive electric charge and negatively charged electrons. The nuclear charge of any chemical element is equal to the product of Z by e, where Z is the ordinal number of this element in the periodic table of chemical elements, e is the value of the elementary electric charge.
Electron - the smallest particle of a substance with a negative electric charge e = 1.6 · 10 -19 pendants, taken as an elementary electric charge. Electrons, rotating around the nucleus, are located on the electron shells of K, L, M, etc. K is the shell nearest to the nucleus. The size of an atom is determined by the size of its electron shell. An atom can lose electrons and become a positive ion or attach electrons and become a negative ion. The ion charge determines the number of electrons lost or attached. The process of converting a neutral atom into a charged ion is called ionization.
Atomic nucleus   (the central part of the atom) consists of elementary nuclear particles - protons and neutrons. The radius of the nucleus is about a hundred thousand times smaller than the radius of an atom. The density of the atomic nucleus is extremely high. Protons   - stable elementary particles having a single positive electric charge and mass, 1836 times greater than the mass of an electron. The proton is the nucleus of the atom of the lightest element - hydrogen. The number of protons in the nucleus is Z. Neutron- a neutral (no electric charge) elementary particle with a mass very close to the proton mass. Since the mass of the nucleus consists of the mass of protons and neutrons, the number of neutrons in the nucleus of an atom is A – Z, where A is the mass number of a given isotope (see the Periodic Table of Chemical Elements). The proton and neutron that make up the nucleus are called nucleons. In the nucleus nucleons are connected by special nuclear forces.
  In the atomic nucleus there is a huge supply of energy that is released during nuclear reactions. Nuclear reactions occur in the interaction of atomic nuclei with elementary particles or with the nuclei of other elements. As a result of nuclear reactions, new nuclei are formed. For example, a neutron can pass into a proton. In this case, a beta particle is ejected from the nucleus, i.e. an electron.
  The transition in the proton nucleus to the neutron can be done in two ways: either a particle with a mass equal to the electron mass but with a positive charge, called a positron (positron decay), is emitted from the nucleus, or the nucleus captures one of the electrons from the K-shell (K - capture).
  Sometimes the nucleus that is formed has an excess of energy (is in an excited state) and, passing into the normal state, releases extra energy in the form of electromagnetic radiation with a very small wavelength - gamma radiation. The energy released during nuclear reactions is practically used in various industries.

Molecule (French molecule, from Latin moles - mass) is the smallest particle of a substance capable of independent existence, possessing its chemical properties.
  The study of the structure and properties of molecules has acquired exceptional interest for understanding the submicroscopic structure of cells and tissues, as well as the mechanism of biological processes at the molecular level. Great success in studying the structure of M. and, in particular, M. such biopolymers as proteins and nucleic acids, showed that the most important functions of these substances in organisms are carried out at the level of individual molecules and therefore should be investigated as molecular phenomena. It has been established, for example, that such protein functions as enzymatic, structural, contractile, immune, transport (reversible binding and transfer of vital substances) are played out at the molecular level and are directly determined by the structure and properties of M. of these substances. The heredity and variability of organisms are associated with the special structure and properties of M. nucleic acids, which contain all the genetic information necessary for the synthesis of body proteins. Small deviations in the structure or composition of molecules of a number of biologically important substances or changes in the molecular mechanism of certain metabolic processes cause a number of diseases (for example, sickle cell anemia, hereditary galactosemia, diabetes mellitus, etc.), called molecular diseases.
  The molecule of each substance consists of a certain number of atoms (see) of one chemical element (simple substance) or various elements (complex substance) united by chemical (valence) bonds. The composition of M. is expressed by a chemical formula in which the signs of the elements indicate the type of atoms forming M. And the numbers on the bottom right show how many atoms of each element are included in M. So, from the chemical formula of glucose SvN12Oe it follows that M. of glucose consists of 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms. Molecules of inert gases and vapors of some metals are monatomic. These are the simplest M. The most complex are M. proteins (see), nucleic acids (see), and other biopolymers consisting of many thousands of atoms.

All substances consist of tiny particles - atoms. Atoms combine to form molecules, the largest of which have a complex structure consisting of thousands of atoms.

The fact that everything is made of particles, knew the ancient Greeks. About 420 BC. e. philosopher Democritus supported the hypothesis that matter consists of tiny, indivisible particles. In Greek, atomos means "indivisible," so these particles are called atoms.

Other philosophers held a different point of view, and in the 4th century BC. e. Aristotle expressed his support for the opinion that matter consists of various combinations of the so-called four elements - earth, air, fire and water. This idea was widely adopted and formed the basis of alchemy, a primitive form of chemistry that prevailed in science until the seventeenth century.

One of the main tasks of alchemy was to create a "elixir of life" - a drug that would allow a person to live forever. The other was to create wealth by converting ordinary metals to gold. Many alchemists claimed that they had accomplished these tasks, but none of them had achieved real success.

Revolution in science

Some scientists continued to adhere to the opinion that matter consists of atoms, but only at the beginning of the 19th century experimental data were obtained confirming this theory. English chemist and writer John Dalton conducted experiments with gases and studied the ways of their connection. So, he found that oxygen and hydrogen, forming water, always combine in the same proportions by weight. Other scientists also encountered similar data, but it was Dalton who first realized their significance. He concluded that substances are composed of atoms, and that all atoms of a simple substance have the same mass. When simple substances are combined, the number of connecting atoms are in a certain constant proportion. Dalton's Atomist explained why substances are combined in a constant mass proportion, and also provided the basis for a detailed study of matter. Substances consist of atoms, and what are atoms made of? The first clues to this mystery appeared at the end of the 19th century, when researchers studied the passage of electricity through discharge tubes containing rarefied air. Sometimes the walls of the tube emitted green light when high voltage was applied to two metal plates - electrodes. The luminescence occurred when invisible rays from a negative electrode, or cathode, hit the tube walls.

In the 1890s, the English physicist J. Thomson proved that these cathode rays (as they were then called) are nothing but streams of negatively charged particles. It was assumed that these particles emanate from the atoms, although their arrangement inside the atoms remained unclear. Thomson suggested that the atom might be similar to Christmas pudding, in which a large but lightweight, positively charged sphere is dotted with numerous negatively charged particles (electrons). However, various experiments on the study of the structure of the atom have proved that this is an absolutely erroneous theory.

Atomic structure

In 1911, Ernest Rutherford, a British physicist, a native of New Zealand, who worked with Thomson, proposed the structure of the atom, which really explains its behavior during experiments. Rutherford suggested that the center (or nucleus) of an atom has a positive charge and a relatively large mass, and extremely light and negatively charged electrons rotate around the nucleus.

However, Rutherford did not realize that there are usually both positively charged and neutral particles in the atomic nucleus. The existence of positively charged particles was recognized in 1920, and they were called protons. In 1932, English physicist James Chadwick discovered uncharged particles and called them neutrons. As a result, the picture of the structure of the atom was completed and has since been the basis of our understanding of matter.

Items

Any substance in which all atoms have the same number of protons is called an element. The number of protons in each atom is the atomic number of the element. There are 92 natural elements, their atoms have from 1 to 92 protons. In addition, some other elements with an even greater number of protons in an atom can be obtained using a device called an elementary particle accelerator. Natural elements include iron, mercury and hydrogen.

In many substances, atoms are combined into groups called molecules. So, hydrogen gas consists of molecules, each of which contains two hydrogen atoms. Often, however, substance molecules consist of atoms of more than one element. Such substances are called compounds. For example, water is a compound where each molecule consists of two hydrogen atoms and one oxygen atom. Many molecules have a much larger number of atoms. Some protein molecules are complex compounds of several thousand atoms. Some natural elements are found only in compounds. So, sodium is a metal that combines so easily with other substances that it cannot be found in its pure form. It is widely known in combination with chlorine in the form of sodium chloride - sodium chloride.

Atoms in molecules bind in different ways, while they share electrons between themselves or exchange them. Two simple types of chemical bonds are covalent and ionic.

Covalent bonding occurs when atoms have common electrons. So, a hydrogen gas molecule consists of two hydrogen atoms linked by a covalent bond. The single electron of each hydrogen atom revolves around the nuclei of both atoms, linking them together.

In the case of an ionic bond, one atom transfers electrons to another atom. The result is an electric force that binds the atoms together. As a rule, the number of positively charged protons and negatively charged electrons in an atom is the same. Their positive and negative charges counterbalance each other, and therefore the atom has no common charge. However, in an atom donating electrons, an excess of positive charge is created, and an atom receiving electrons acquires a total negative charge. Such charged atoms are called ions. Ions of opposite charges attract each other, and it is this electrical attraction that holds the atoms together in the ionic bond. For example, a salt molecule is formed by an ionic bond when a sodium atom transfers an electron to a chlorine atom.

All atoms of one substance have the same number of protons, but a different number of neutrons. Thus, in carbon, the nucleus of most atoms contains six neutrons, but approximately in every hundredth of them there are seven neutrons. These different types of atoms of the same element are called isotopes. All isotopes of this element have the same chemical properties - they all combine with other substances and form the same chemical compounds. But the individual physical properties of isotopes are different - for example, they have different freezing or boiling points.

Speaking about a particular isotope of an element, scientists call it a mass number. For example, carbon-12 is a common natural carbon isotope. Its atom contains six protons and six neutrons. A more rare natural isotope, in the nucleus of each atom of which there is an extra neutron, is called carbon-13.

Atomic weight

A proton and a neutron have almost the same mass, which is more than 1,800 times the mass of an electron. Therefore, when it comes to the mass of an atom, as a rule, it will not be an error to refer to its mass number.

The atomic weight of an element, or its relative atomic mass, is usually the average mass of a mixture of isotopes found in nature. The molecular weight of a substance, or its relative molecular weight, is the sum of the atomic weights of all atoms in one molecule of a given substance.

Polysyllabic atom

Since then, scientists have experimented with accelerators discovered hundreds of other types of particles in atoms. But, fortunately, a simple atomic model is sufficient to explain most of the properties of matter.