level of knowledge about the structure of atoms and molecules in the XIX century can not explain the reason for which the atoms form a certain number of connections with other particles.But the ideas of scientists ahead of their time, and the valence is still being studied as one of the basic principles of chemistry.
From the history of the concept of "valence of the chemical elements»
eminent English chemist Edward Frankland XIX century coined the term "connection" to the academic community to describe the interaction of atoms with each other.Scientists noticed that some chemical elements form compounds with the same amount of the other atoms.For example, nitrogen attaches three hydrogen atoms in the ammonia molecule.
In May 1852 Frankland put forward the hypothesis that there is a specific number of chemical bonds that atom can form with other tiny particles of matter.Frankland used the phrase "coupling strength" to describe what would later be called the valence.British chemist established as the chemical bonds form atoms of certain elements known in the middle of the XIX century.Work Frankland was an important contribution to modern structural chemistry.
Development views
German chemist FAKekule proved in 1857 that carbon is chetyrehosnovnym.In its simplest compound - methane - occur due to the four hydrogen atoms.The term "core" is used to denote a scientist properties of elements to attach a fixed number of other particles.In Russia, the data on the structure of matter systematized AMButlerov (1861).Further development of the theory of chemical bonding obtained through the teachings of the periodic variation of the properties of the elements.Its author - another outstanding Russian chemist Dmitri Mendeleev.He proved that the valence of the chemical elements in the compounds, and other properties are due to the position they occupy in the periodic system.
Graphical valence and chemical bonding
The possibility of visualizing molecules - one of the great things about the theory of valence.The first models appeared in 1860, and since 1864 the structural formulas used is a circle with a chemical mark inside.Between characters atoms dash is a chemical bond, and the number of lines is equal to the valence.In those same years it had made the first sharosterzhnevye model (see. Photo on the left).In 1866 Kekule proposed stereochemical drawing carbon atoms in the form of a tetrahedron, which he included in his textbook "Organic Chemistry".
valence of the chemical elements, and the emergence of relationships studied G. Lewis, who published his works in 1923 after the discovery of the electron.So called negatively charged tiny particles that are part of the shells of atoms.In his book, Lewis used a point around four sides of the chemical symbol for the display of the valence electrons.
valence of hydrogen and oxygen
Before the creation of the periodic system of the valence of the chemical elements in the compounds used to be compared with the atoms to which it is known.The standards were chosen as hydrogen and oxygen.Other elements attracted a certain number of atoms substituted for the H and O.
this way determines the properties of compounds of monovalent hydrogen (the valence of the second element denotes the Roman numeral):
- HCl - chlorine (I):
- H2O - oxygen (II);
- NH3 - Nitrogen (III);
- CH4 - carbon (IV).
to oxide K2O, CO, N2O3, SiO2, SO3 oxygen determined by the valence of metals and nonmetals, doubling the number of atoms attachable O. following values are obtained: K (I), C (II), N (III), Si (IV), S (VI).
How to determine the valence of the chemical elements
There are laws of formation of chemical bonds with the common electron pairs:
- typical valence of hydrogen - I.
- usual valence of oxygen - II.For
- elements nonmetals lower valence can be determined by the formula 8, - the group number, in which they are in the periodic system.Major, if it can be determined by the group number.For elements
- secondary subgroups maximum possible valency is the same as the number of groups in the periodic table.
Determination of the valence of the chemical elements in the formula of the compound is carried out using the following algorithm:
- Record on top of the chemical symbol for the known value of one of the elements.For example, the valence of oxygen is Mn2O7 II.
- Calculate the total amount, which must be multiplied by the number of valence of the atoms of the same chemical element in the molecule: 2 * 7 = 14.
- determine the valence of the second element for which it is known.Divide obtained in Sec. 2 value by the number of Mn atoms in the molecule.
- 14: 2 = 7. The valence of the manganese oxide in its highest - VII.
constant and variable values of the valence
valence of hydrogen and oxygen are different.For example, sulfur in the compound of divalent H2S, and in the formula SO3 - hexavalent.Carbon monoxide, oxygen reacts with the CO and CO2 dioxide.In the former compound II equal to the valency of C, and in the second - IV.The same value in methane CH4.
Most of the exhibits are not constant and variable valence, such as phosphorus, nitrogen, sulfur.The search for the main causes of this phenomenon led to the chemical bond theory, representations of the valence shell electron molecular orbitals.The existence of different values of the same properties has been explained from the standpoint of the structure of atoms and molecules.
Modern conceptions of valence
All atoms consist of a positive nucleus surrounded by negatively charged electrons.The outer shell of which they form, is unfinished.The complete structure of the most stable, it contains 8 electrons (octet).The emergence of the chemical bond with the common pairs of electrons leads to the energetically favorable state of the atom.
Rule for the formation of compounds is the completion of the shell of electrons by receiving a return of unpaired - depending on what the process is easier to pass.If an atom provides for the formation of chemical bonds negative particles that do not have a pair, the bonds it forms as much as he has unpaired electrons.According to modern concepts, the valence of atoms of chemical elements - is the ability to produce a certain number of covalent bonds.For example, a molecule of hydrogen sulfide H2S becomes sulfur valence II (-), because each atom participates in the formation of two electron pairs.The sign "-" indicates the attraction of the electron pair to the more electronegative element.At least electronegative valence appends the value "+".
When the donor-acceptor mechanism in the process of participating electron pairs of one element and the free valence orbitals of the other.
dependence of the valence of the structure of the atom
Consider the example of carbon and oxygen, it depends on the structure of matter valence of the chemical elements.Periodic Table gives an overview of the main characteristics of the carbon atom:
- chemical symbol - C;
- item number - 6;
- nuclear charge - 6;
- protons in the nucleus - 6;
- electron - 6, including 4 external, 2 of which form a pair, two - unpaired.
If the carbon atom in CO monoookside two forms of communication, is in its use comes only 6 negative particles.To purchase octets needed to form a pair of 4 external negative particles.Carbon has a valence of IV (+) dioxide and IV (-) in methane.
Number of oxygen - 8 valence shell consists of six electrons, two of them do not form a pair and are involved in chemical bonding and interaction with other atoms.Typical oxygen valence - II (-).
Valence and the degree of oxidation
In many cases it is more convenient to use the term "degree of oxidation".So they called the charge of the atom, which he would acquire if all binding electrons are passed to the element, which has a higher value elektroootritsatelnosti (EO).The oxidation number of the simple substance is zero.By the oxidation state of an element is added to a EO "-" sign, the less electronegative - "+".For example, a typical main group metal oxidation state and charges of ions, equal to the group number with a "+".In most cases the valency and oxidation state of atoms in the same compound is numerically identical.Only when interaction with more electronegative atoms positive oxidation state, with elements that have lower EO - negative.The concept of "valence of" often applies only to the substance of the molecular structure.
