Chinese name: * * valence compound mbth: covalent compound classification: chemical properties: non-conductive in molten state, two-point properties, chemical properties The essence of chemical changes is the breaking of old bonds and the formation of new bonds. In chemical reactions, there are two ways to break the valence bond, which have an important influence on chemical reactions, especially in organic chemistry. When the valence bond of homolysis and free radical reaction is homolysis, the bonding electrons are evenly distributed to two atoms (groups), and the single electron atom (group) produced by homolysis is called free radical, which is represented by "R". Free radicals are reactive and can participate in chemical reactions. Free radical reactions are generally carried out under the action of light or heat. Heterocleavage and ion reaction * * * Valence bonds generate positive and negative ions when heterocleavage occurs, for example, hydrogen chloride is ionized into hydrogen ions and chloride ions in water. Carbonated cations and anions produced by heterocleavage of valence bonds of organic compounds are active species in organic reactions, which often participate in the reaction at the moment of production, but their existence can be proved. The reaction caused by heterocleavage of valence bond is called ionic reaction, which can be divided into electrophilic reaction and nucleophilic reaction. The main feature of saturation is that in the process of forming valence bonds, because the number of unpaired electrons provided by each atom is certain, an unpaired electron of an atom can not be paired with other electrons after being paired with unpaired electrons of other atoms, that is, the total number of valence bonds formed by each atom is certain, which is * * * valence bond saturation. * * * The saturation of valence bond determines the quantitative relationship when various atoms form molecules, which is one of the internal reasons of the law of fixed ratio. Directionality Except that the S orbital is spherical, all other atomic orbitals have their fixed extension directions, so when valence bonds are formed, orbital overlap also has a fixed direction, and valence bonds also have their directionality, and the direction of valence bonds determines the configuration of molecules. Compounds with two properties, such as hydrogen chloride, are called * * * valence compounds. Such as water H2O, carbon dioxide CO2, NH3, etc. Are common compounds with valence of * * *. * * * valence compounds are generally molecular crystals, such as aluminum chloride AlCl3, which are * * * valence compounds. (Mercury chloride and silver iodide are also * * * valence compounds) * * * valence compounds must have at least one * * valence bond, not ionic bonds! * * * Valence compounds are mostly molecular crystals, so they are also called molecular compounds. Molecular compounds must be * * * valence compounds. * * * Valence compounds include molecular compounds and atomic compounds. For example, silicon dioxide is an atomic compound, but it is also a valence compound. In addition, the force between most rare gases is van der Waals force, and the molecules of bivalent compounds are formed by * * * electron pairs between atoms. When atoms of two nonmetallic elements (or inactive metal elements and nonmetallic elements) combine, each atom emits one or more electrons to form electron pairs, which are attracted by the nuclei of the two elements and occupied by the two elements, so that the atoms of the two elements form compound molecules. For example, hydrogen chloride is a compound molecule composed of hydrogen atoms and chlorine atoms, each of which has an outermost electron to form a pair of electrons. Non-metallic hydrides (such as HCl, H2O, NH3, etc. ), nonmetallic oxides (such as CO2, SO3, etc. ), anhydrous acid (such as H2SO4, HNO3, etc. ) and most organic compounds (such as methane, alcohol, sucrose, etc. ) is a * * * valence compound. When most compounds with valence of * * * are in solid state, their melting point and boiling point are low, and their hardness is small. When atoms of two nonmetallic elements form molecules, because both atoms have a tendency to form a stable structure of 8 electrons by acquiring electrons, their ability to acquire electrons is almost the same, and no one can take away the other's electrons. In this way, two atoms can only provide one electron each to form an electron pair, which moves in the outer space of two atoms, and the electron is negatively charged and the nucleus is positively charged. The nuclei of two atoms attract electron pairs at the same time, producing a force, thus forming a molecule. Because the two atoms have different abilities to attract electrons, the electron pair of * * * always favors the side with strong electron ability. The atom on this side is slightly negatively charged, and the atom on the other side is slightly positively charged. As a whole, the molecule is still electrically neutral. Typical valence compounds are water, hydrogen chloride and carbon dioxide. * * * electron pairs are always biased towards oxygen atoms and away from hydrogen atoms. Valence bonds formed between atoms of the same kind of different elements are called valence bonds. Generally, valence compounds are all valence compounds. * * * Valence compounds generally have low hardness and low melting point. Some simple molecules are also formed by electron pairs. For example, a chlorine molecule is formed by two chlorine atoms, each chlorine atom provides an electron to form an electron pair, and the electron pair is simultaneously acted on by two nuclei to form a chlorine molecule. Because atoms of the same kind have similar ability to attract electrons, electron pairs are not biased to either side. The valence bond formed between atoms of the same element is called nonpolar valence bond. * * * uses the electron pair in the middle of two bonding atoms and does not favor either side. In early ancient Greece, before chemistry was separated from natural philosophy, atomists had the most primitive concept of chemical bond. Empedocles thinks that the world consists of four elements: air, water, earth and fire. When these four elements split under the action of "love" and "hate" and regrouped into a new arrangement, things changed qualitatively. This force can be regarded as the earliest chemical bond idea. Later, democritus, an atomist, imagined that there was a kind of "hook" between atoms, or a rough surface, which made them stick together when they collided with each other to form a stable aggregate. Democritus's concept of chemical bond is more advanced than previous natural philosophers, and he eliminated the idealistic factors in this concept. Glauber in the Middle Ages put forward the view that the same kind of substances repel each other and the different kinds of substances repel each other. Since then, there has been an affinity theory about the combination of matter, which holds that the particles of matter have affinity, thus attracting each other and combining together. In a word, people's hazy understanding of chemical bonds inspired later chemists. In the18th century, the concept of phologiston entered the field of chemistry and was accepted by senior chemists such as Ernst Starr, Henry cavendish and joseph priestley. At that time, Newtonian mechanics had been put forward, and they hoped to combine the interaction between atoms with Newtonian mechanics to give a classical physical explanation, but because of the conditions at that time, this was undoubtedly impossible. 19 16 years, German chemist Albray A.Kossel came to the conclusion after investigating a large number of facts: the atoms of any element should make the outermost layer satisfy the 8-electron stable structure, but Kosel only explained the formation process of ionic compounds, but did not explain the formation of valence bonds. 19 19 chemist irving langmuir first used valence to describe the bonding process between atoms (original text). We will deny the number of electron pairs shared by a given atom and its neighboring atoms through $ TERM co valence (we should use valence). The word "valence" means * *. ) 1922, Niels n Bohr re-examined Rutherford's nuclear model from the perspective of quantization, which provided a brand-new platform for chemists to understand chemical bonds. He believes that electrons should be in a certain orbit and can jump between different orbits, and the steady-state transition can well explain every spectral line of the hydrogen atom spectrum. 1923, American chemist gilbert G.N.Lewis developed Kosel's theory and put forward the valence bond electron pair theory. Lewis assumes that the electrons of one atom and another atom in the molecule form chemical bonds between atoms in the form of "electron pairs". At that time, this was a hypothesis contrary to the orthodox theory, because Coulomb's law showed that two electrons were mutually exclusive, but Lewis's idea was quickly accepted by the chemical community, which led to the hypothesis that the electron spins between atoms were opposite. 1924, Louis Broglie put forward the wave-particle duality hypothesis, established the mathematical model of the atom, and described the electron as a three-dimensional waveform. Mathematically, the exact values of position and momentum cannot be obtained at the same time. 1926, Schrodinger put forward the wave equation of quantum mechanics, which can be directly used to explain the "formation" and "fracture" of chemical bonds, and became the initial beginning of quantum chemistry. 1927, W. H. Hai and F. London treated hydrogen molecules with quantum mechanics, calculated the wave function of hydrogen molecular system with approximate method, and solved the valence bond problem with quantum mechanics for the first time. The valence bond theory was born in the popularization of this method, and their method of studying valence bond is called HL method. 1928, Enrica Fermi proposed a single electron density model based on Poisson distribution, trying to solve the problem of atomic structure. After that, Douglas Rayner Hartree decomposed the Hamiltonian of the system into the simple sum of several single-electron Hamiltonians by iterative method, and then expressed the multi-electron wave function of the system as the product of single-electron wave functions, improved this model and proposed Hartree equation. In 1930, Hartley students Fogg and John Slater perfected Hartley equation, which is called Hartley-Fogg equation (HF). In the early 1950s, Slater obtained the approximate wave function of HF: Hartley-Fokker-Slater equation (HFS). In 1963, Herman and skillman applied HFS to the atomic function of the ground state. 1950, clemons further proposed to linearly expand the molecular orbitals in the equation with the atomic orbitals of the constituent molecules, and developed the famous RHF equation. 1964, computer chemist E.Clementi published a large number of RHF wave functions, as well as this equation and its follow-up. 1929, Bate and others put forward the coordination field theory, which was first used to discuss the energy level splitting of transition metal ions in the crystal field, and then combined with the molecular orbital theory to develop into a modern coordination field theory. 1930, American chemist linus L.C.Pauling put forward the orbital hybridization theory when studying the regular tetrahedron configuration of carbon. He believes that orbits with similar energy levels can hybridize to form new degenerate orbits when excited. The theoretical basis is the wave-particle duality of electrons, and waves can be superimposed. He calculated the shapes of various hybrid orbitals and won the Nobel Prize in chemistry for his outstanding contribution to the valence bond theory. In 1932, F.Hund divided the valence bond of * * * into three types: σ bond, π bond and δ bond, which further systematized the valence bond theory and organically combined it with the classical valence bond theory. In the same year, American chemist Roberts. Mulligan put forward the molecular orbital theory. It is believed that the electrons in a compound do not belong to an atom, but move in the whole molecule. His method is too far away from classical chemistry, and the calculation is very complicated, so it is not accepted by the chemical community for the time being. With the perfection of Robert A.Millikan, Philip Leonard (Philipp Lenard) and Erich Hückel (Erich Hückel), it is gradually recognized in the field of chemistry. 1940, H.Sidgwick and Thomas A.Powell put forward a simple theoretical model to predict the three-dimensional structure of simple molecules or ions on the basis of summarizing experimental facts. This theoretical model was put forward by R.J.Gillespie and R. S. Jesper Gunnar Fernando R. S. Nyholm in 1950s, and it was named valence electron pair repulsion theory, abbreviated as VSEPR. Combined with orbital hybridization theory, VSEPR can semi-quantitatively infer the bonding mode and molecular structure of molecules. In 195 1, Kenichi Fukui put forward the frontier orbital theory, holding that the molecular orbital with the highest energy and the molecular orbital with the lowest energy that are not occupied by electrons are the key to determine the chemical reaction of a system, while the molecular orbital with other energies has little influence on the chemical reaction, which can be ignored for the time being. HOMO and LUMO are the so-called frontier orbits. 1965, American chemists Rober B.Woodward and Hoffman proposed the conservation of molecular orbital symmetry with reference to Kenichi Fukui's frontier orbital theory. The molecular orbital theory has made new progress. Due to the rapid development of computer technology and the application of Monte Carlo method, quantum chemistry and computer chemistry are changing with each passing day, and the calculation of molecular structure is becoming more and more accurate. During this period, a large number of outstanding chemists were born. It is estimated that there will be a new breakthrough in quantum chemistry in the mid-20th century.