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.
An electron is 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.
The atomic nucleus (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 are 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. The neutron is a neutral (not having an electric charge) elementary particle with a mass very close to the mass of the proton. 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.Go
The atomic nucleus has an enormous amount 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, that is, 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 - 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.
Atom (Greek atomos - indivisible) is the smallest particle of a chemical element, possessing its chemical properties. Each element consists of atoms of a certain type. The composition of the atom consists of a nucleus, carrying a positive electric charge, and negatively charged electrons (see), forming its electron shells. The value of the electric charge of the nucleus is Ze, where e is the elementary electric charge equal in magnitude to the electron charge (4.8 · 10 -10 el.-stead.), And Z is the atomic number of this element in the periodic system of chemical elements (see .). Since the non-ionized atom is neutral, the number of electrons entering it is also equal to Z. The nucleus (see Nucleus atomic) contains nucleons, elementary particles with a mass approximately 1840 times greater than the mass of an electron (equal to 9.1 · 10 - 28 g), protons (see), positively charged, and having no charge neutrons (see). The number of nucleons in the nucleus is called the mass number and is denoted by the letter A. The number of protons in the nucleus, equal to Z, determines the number of electrons entering the atom, the structure of the electron shells and the chemical properties of the atom. The number of neutrons in the nucleus is A – Z. Isotopes are called species of the same element, the atoms of which differ from each other in mass number A, but have the same Z. Thus, in the nuclei of atoms of different isotopes of one element there is a different number of neutrons with the same number of protons. In the designation of isotopes, the mass number A is written on top of the element symbol, and the atomic number is on the bottom; for example, oxygen isotopes are designated:Go
The size of an atom is determined by the size of the electron shells and amounts to about 10 -8 cm for all Z. Since the mass of all electrons in an atom is several thousand times smaller than the mass of the nucleus, the mass of the atom is proportional to the mass number. The relative mass of the atom of this isotope is determined by the mass of the atom of the carbon isotope C 12 , taken as 12 units, and is called the isotopic mass. It turns out to be close to the mass number of the corresponding isotope. The relative weight of an atom of a chemical element is the average (taking into account the relative abundance of isotopes of this element) isotopic weight and is called atomic weight (mass).
An atom is a microscopic system, and its structure and properties can be explained only with the help of quantum theory, created mainly in the 20s of the 20th century and intended to describe phenomena of atomic scale. Experiments have shown that microparticles - electrons, protons, atoms, etc., - except for corpuscular ones, have wave properties that manifest themselves in diffraction and interference. In quantum theory, a wave field characterized by a wave function (Ψ-function) is used to describe the state of micro-objects. This function determines the probabilities of possible states of a micro-object, i.e., it characterizes the potential possibilities of manifestation of one or another of its properties. The law of variation of the function Ψ in space and time (the Schrödinger equation), which makes it possible to find this function, plays the same role in quantum theory as in the classical mechanics of Newton's laws of motion. Solving the Schrödinger equation in many cases leads to discrete possible states of the system. For example, in the case of an atom, a series of wave functions is obtained for electrons corresponding to different (quantized) energy values. The system of energy levels of the atom, calculated by the methods of quantum theory, has received brilliant confirmation in spectroscopy. The transition of an atom from the ground state corresponding to the lowest energy level E 0 to any of the excited states E i occurs when a certain portion of the energy E i - E 0 is absorbed. An excited atom enters a less excited or ground state, usually with the emission of a photon. The photon energy hv is equal to the difference in the atomic energy in two states: hv = E i - Е k where h is the Planck constant (6.62 · 10 -27 erg · s), v is the frequency of light.
In addition to the atomic spectra, quantum theory allowed us to explain other properties of atoms. In particular, the valence, the nature of the chemical bond and the structure of the molecules were explained, the theory of the periodic system of elements was created.