In chemistry lessons you have already become acquainted with the concept of valency. chemical elements. We have collected all in one place useful information about this question. Use it when you prepare for the State Exam and the Unified State Exam.
Valency and chemical analysis
Valence– the ability of atoms of chemical elements to enter into chemical compounds with atoms of other elements. In other words, it is the ability of an atom to form a certain number of chemical bonds with other atoms.
From Latin the word “valency” is translated as “strength, ability.” A very correct name, right?
The concept of “valence” is one of the basic ones in chemistry. It was introduced even before scientists knew the structure of the atom (back in 1853). Therefore, as we studied the structure of the atom, it underwent some changes.
Thus, from the point of view of electronic theory, valence is directly related to the number of outer electrons of an element’s atom. This means that “valency” refers to the number of electron pairs that an atom has with other atoms.
Knowing this, scientists were able to describe the nature of the chemical bond. It lies in the fact that a pair of atoms of a substance shares a pair of valence electrons.
You may ask, how were chemists of the 19th century able to describe valence even when they believed that there were no particles smaller than an atom? This is not to say that it was so simple - they relied on chemical analysis.
By chemical analysis scientists of the past determined the composition chemical compound: how many atoms of different elements are contained in the molecule of the substance in question. To do this, it was necessary to determine what the exact mass of each element in a sample of pure (without impurities) substance was.
True, this method is not without flaws. Because the valence of an element can be determined in this way only in its simple combination with always monovalent hydrogen (hydride) or always divalent oxygen (oxide). For example, the valency of nitrogen in NH 3 is III, since one hydrogen atom is bonded to three nitrogen atoms. And the valency of carbon in methane (CH 4), according to the same principle, is IV.
This method for determining valency is only suitable for simple substances. But in acids, in this way we can only determine the valency of compounds such as acidic residues, but not of all elements (except for the known valency of hydrogen) individually.
As you have already noticed, valence is indicated by Roman numerals.
Valency and acids
Since the valence of hydrogen remains unchanged and is well known to you, you can easily determine the valence of the acid residue. So, for example, in H 2 SO 3 the valency of SO 3 is I, in HСlO 3 the valency of СlO 3 is I.
In a similar way, if the valence of the acid residue is known, it is easy to write down the correct formula of the acid: NO 2 (I) - HNO 2, S 4 O 6 (II) - H 2 S 4 O 6.
Valency and formulas
The concept of valency makes sense only for substances of a molecular nature and is not very suitable for describing chemical bonds in compounds of a cluster, ionic, crystalline nature, etc.
Indices in the molecular formulas of substances reflect the number of atoms of the elements that make up them. Knowing the valence of elements helps to correctly place the indices. In the same way, by looking at the molecular formula and indices, you can tell the valences of the constituent elements.
You do tasks like this in chemistry lessons at school. For example, having the chemical formula of a substance in which the valency of one of the elements is known, you can easily determine the valence of another element.
To do this, you just need to remember that in a substance of a molecular nature, the number of valences of both elements is equal. Therefore, use the least common multiple (corresponding to the number of free valencies required for the compound) to determine the valence of an element that is unknown to you.
To make it clear, let's take the formula of iron oxide Fe 2 O 3. Here, two iron atoms with valence III and 3 oxygen atoms with valency II participate in the formation of a chemical bond. Their least common multiple is 6.
- Example: you have the formulas Mn 2 O 7. You know the valence of oxygen, it is easy to calculate that the least common multiple is 14, hence the valence of Mn is VII.
In a similar way, you can do the opposite: write down the correct chemical formula of a substance, knowing the valences of its elements.
- Example: to correctly write the formula of phosphorus oxide, we take into account the valency of oxygen (II) and phosphorus (V). This means that the least common multiple for P and O is 10. Therefore, the formula has the following form: P 2 O 5.
Knowing well the properties of elements that they exhibit in various compounds, it is possible to determine their valence even by appearance such connections.
For example: copper oxides are red (Cu 2 O) and black (CuO) in color. Copper hydroxides are colored yellow (CuOH) and blue (Cu(OH) 2).
To make covalent bonds in substances more visual and understandable for you, write them down structural formulas. The lines between the elements represent the bonds (valency) that arise between their atoms:
Valency characteristics
Today, the determination of the valency of elements is based on knowledge of the structure of the outer electronic shells of their atoms.
Valency can be:
- constant (metals of the main subgroups);
- variable (non-metals and metals of secondary groups):
- higher valence;
- lower valence.
The following remains constant in various chemical compounds:
- valence of hydrogen, sodium, potassium, fluorine (I);
- valency of oxygen, magnesium, calcium, zinc (II);
- valence of aluminum (III).
But the valence of iron and copper, bromine and chlorine, as well as many other elements changes when they form various chemical compounds.
Valence and electron theory
Within the framework of electronic theory, the valence of an atom is determined based on the number of unpaired electrons that participate in the formation of electron pairs with electrons of other atoms.
Only electrons located in the outer shell of an atom participate in the formation of chemical bonds. Therefore, the maximum valence of a chemical element is the number of electrons in the outer electron shell of its atom.
The concept of valency is closely related to the Periodic Law, discovered by D. I. Mendeleev. If you look carefully at the periodic table, you can easily notice: the position of an element in the periodic system and its valency are inextricably linked. The highest valence of elements that belong to the same group corresponds to the ordinal number of the group in the periodic table.
You will find out the lowest valency when you subtract the group number of the element that interests you from the number of groups in the periodic table (there are eight of them).
For example, the valency of many metals coincides with the numbers of the groups in the table of periodic elements to which they belong.
Table of valency of chemical elements
|
Serial number chem. element (atomic number) |
Name |
Chemical symbol |
Valence |
| 1 | Hydrogen Helium Lithium Beryllium Carbon Nitrogen / Nitrogen Oxygen Fluorine Neon / Neon Sodium/Sodium Magnesium / Magnesium Aluminum Silicon Phosphorus / Phosphorus Sulfur/Sulfur Chlorine Argon / Argon Potassium/Potassium Calcium Scandium / Scandium Titanium Vanadium Chrome / Chromium Manganese / Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic/Arsenic Selenium Bromine Krypton / Krypton Rubidium / Rubidium Strontium / Strontium Yttrium / Yttrium Zirconium / Zirconium Niobium / Niobium Molybdenum Technetium / Technetium Ruthenium / Ruthenium Rhodium Palladium Silver Cadmium Indium Tin/Tin Antimony / Antimony Tellurium / Tellurium Iodine / Iodine Xenon / Xenon Cesium Barium / Barium Lanthanum / Lanthanum Cerium Praseodymium / Praseodymium Neodymium / Neodymium Promethium / Promethium Samarium / Samarium Europium Gadolinium / Gadolinium Terbium / Terbium Dysprosium / Dysprosium Holmium Erbium Thulium Ytterbium / Ytterbium Lutetium / Lutetium Hafnium / Hafnium Tantalum / Tantalum Tungsten/Tungsten Rhenium / Rhenium Osmium / Osmium Iridium / Iridium Platinum Gold Mercury Thalium / Thallium Lead/Lead Bismuth Polonium Astatine Radon / Radon Francium Radium Actinium Thorium Proactinium / Protactinium Uranium / Uranium |
H | I (I), II, III, IV, V I, (II), III, (IV), V, VII II, (III), IV, VI, VII II, III, (IV), VI (I), II, (III), (IV) I, (III), (IV), V (II), (III), IV (II), III, (IV), V (II), III, (IV), (V), VI (II), III, IV, (VI), (VII), VIII (II), (III), IV, (VI) I, (III), (IV), V, VII (II), (III), (IV), (V), VI (I), II, (III), IV, (V), VI, VII (II), III, IV, VI, VIII (I), (II), III, IV, VI (I), II, (III), IV, VI (II), III, (IV), (V) No data No data (II), III, IV, (V), VI |
Those valences that the elements possessing them rarely exhibit are given in parentheses.
Valency and oxidation state
Thus, speaking about the degree of oxidation, it is meant that an atom in a substance of ionic (which is important) nature has a certain conventional charge. And if valence is neutral characteristic, then the oxidation state can be negative, positive or equal to zero.
It is interesting that for an atom of the same element, depending on the elements with which it forms a chemical compound, the valence and oxidation state can be the same (H 2 O, CH 4, etc.) or different (H 2 O 2, HNO 3 ).
Conclusion
By deepening your knowledge of the structure of atoms, you will learn more deeply and in more detail about valency. This description of chemical elements is not exhaustive. But it has great practical significance. As you yourself have seen more than once, solving problems and conducting chemical experiments in your lessons.
This article is designed to help you organize your knowledge about valence. And also remind you how it can be determined and where valence is used.
We hope you find this material useful in preparing your homework and self-preparing for tests and exams.
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Looking at the formulas of various compounds, it is easy to notice that number of atoms the same element in molecules various substances not the same. For example, HCl, NH 4 Cl, H 2 S, H 3 PO 4, etc. The number of hydrogen atoms in these compounds varies from 1 to 4. This is characteristic not only of hydrogen.
How can you guess which index to put next to the designation of a chemical element? How are the formulas of a substance made? This is easy to do when you know the valency of the elements that make up the molecule of a given substance.
– is the property of an atom of a given element to attach, hold, or replace in chemical reactions a certain number of atoms of another element. The unit of valency is the valence of a hydrogen atom. Therefore, sometimes the definition of valence is formulated as follows: valence – This is the property of an atom of a given element to attach or replace a certain number of hydrogen atoms.
If one hydrogen atom is attached to one atom of a given element, then the element is monovalent, if two – divalent and etc. Hydrogen compounds are not known for all elements, but almost all elements form compounds with oxygen O. Oxygen is considered to be constantly divalent.
Constant valency:
I –
H, Na, Li, K, Rb, Cs
II –
O, Be, Mg, Ca, Sr, Ba, Ra, Zn, Cd
III –
B, Al, Ga, In
But what to do if the element does not combine with hydrogen? Then the valence of the required element is determined by the valence of the known element. Most often it is found using the valence of oxygen, because in compounds its valency is always 2. For example, it is not difficult to find the valence of elements in the following compounds: Na 2 O (valence of Na – 1, O – 2), Al 2 O 3 (valence of Al – 3, O – 2).
The chemical formula of a given substance can only be compiled by knowing the valency of the elements. For example, it is easy to create formulas for compounds such as CaO, BaO, CO, because the number of atoms in the molecules is the same, since the valences of the elements are equal.
What if the valences are different? When do we act in such a case? Need to remember next rule: in the formula of any chemical compound, the product of the valence of one element by the number of its atoms in the molecule is equal to the product of the valence by the number of atoms of another element. For example, if it is known that the valence of Mn in a compound is 7, and O – 2, then the formula of the compound will look like this: Mn 2 O 7.
How did we get the formula?
Let's consider an algorithm for compiling formulas by valence for compounds consisting of two chemical elements.
There is a rule that the number of valencies of one chemical element is equal to the number of valencies of another. Let us consider the example of the formation of a molecule consisting of manganese and oxygen.
We will compose in accordance with the algorithm:
1. We write down the symbols of chemical elements next to each other:
2.
We put the numbers of their valency over the chemical elements (the valence of a chemical element can be found in the table of the periodic system of Mendelev, for manganese –
7, at oxygen –
2.
3. Find the least common multiple ( smallest number, which is divisible by 7 and 2 without a remainder). This number is 14. We divide it by the valences of the elements 14: 7 = 2, 14: 2 = 7, 2 and 7 will be the indices for phosphorus and oxygen, respectively. We substitute indices.
Knowing the valence of one chemical element, following the rule: valence of one element × the number of its atoms in the molecule = valence of another element × the number of atoms of this (other) element, you can determine the valence of another.
Mn 2 O 7 (7 2 = 2 7).
The concept of valence was introduced into chemistry before the structure of the atom became known. It has now been established that this property of an element is related to the number of external electrons. For many elements, the maximum valence follows from the position of these elements in the periodic table.
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People often hear the word “valence” without fully understanding what it is. So what is valence? Valence is one of the terms that is used in chemical structure. Valence essentially determines the ability of an atom to form chemical bonds. Quantitatively, valency is the number of bonds in which an atom participates.
What is the valence of an element
Valence is an indicator of the ability of an atom to attach other atoms, forming chemical bonds with them inside the molecule. The number of bonds of an atom is equal to the number of its unpaired electrons. These bonds are called covalent.
An unpaired electron is a free electron on the outer shell of an atom that pairs with the outer electron of another atom. Each pair of such electrons is called an “electron”, and each of the electrons is called a valence. So the definition of the word “valency” is the number of electron pairs with which one atom is connected to another atom.
Valency can be schematically depicted in structural chemical formulas. When this is not necessary, simple formulas are used where the valence is not indicated.
The maximum valence of chemical elements from one group of the periodic system of Mendeleev is equal to the serial number of this group. Atoms of the same element can have different valencies in different chemical compounds. The polarity of the covalent bonds that are formed is not taken into account. This is why valence has no sign. Also, valence cannot be a negative value and equal zero.
Sometimes the concept of “valence” is equated with the concept of “oxidation state,” but this is not true, although sometimes these indicators do coincide. Oxidation number is a formal term that refers to the possible charge that an atom would receive if its electron pairs were transferred to more electrically negative atoms. Here the oxidation state may have some sign and is expressed in units of charge. This term is common in Not organic chemistry, because in inorganic compounds It's hard to judge valence. Conversely, in organic chemistry, valency is used because molecular structure It has most of organic compounds.
Now you know what the valency of chemical elements is!
How to determine the valence of chemical elements? This question is faced by everyone who is just starting to get acquainted with chemistry. First, let's find out what it is. Valency can be considered as the property of atoms of one element to hold a certain number of atoms of another element.
Elements with constant and variable valency
For example, from formulas H-O-H it is clear that each H atom is connected to only one atom (in in this case with oxygen). It follows that its valence is 1. The O atom in a water molecule is bonded to two monovalent H atoms, which means it is divalent. Valence values are written in Roman numerals above the symbols of the elements:
The valencies of hydrogen and oxygen are constant. However, there are exceptions for oxygen. For example, in the hydronium ion H3O+, oxygen is trivalent. There are other elements with constant valency.
- Li, Na, K, F – monovalent;
- Be, Mg, Ca, Sr, Ba, Cd, Zn – have a valence of II;
- Al, B are trivalent.
Now let's determine the valency of sulfur in the compounds H2S, SO2 and SO3.
In the first case, one sulfur atom is bonded to two monovalent H atoms, which means its valence is two. In the second example, for one sulfur atom there are two oxygen atoms, which, as is known, is divalent. We obtain a valency of sulfur equal to IV. In the third case, one S atom attaches three O atoms, which means that the valence of sulfur is equal to VI (the valence of atoms of one element multiplied by their number).
As you can see, sulfur can be di-, tetra- and hexavalent:
Such elements are said to have variable valency.
Rules for determining valencies
- The maximum valency for the atoms of a given element coincides with the number of the group in which it is located in the Periodic Table. For example, for Ca it is 2, for sulfur – 6, for chlorine – 7. There are also many exceptions to this rule:
-element of group 6, O, has valency II (in H3O+ – III);
- monovalent F (instead of 7);
-usually di- and trivalent iron, an element of group VIII;
-N can only hold 4 atoms near itself, and not 5, as follows from the group number;
- mono- and divalent copper, located in group I. - The minimum value of valence for elements for which it is variable is determined by the formula: group number in PS - 8. Thus, the lowest valence of sulfur 8 - 6 = 2, fluorine and other halogens - (8 - 7) = 1, nitrogen and phosphorus - (8 – 5)= 3 and so on.
- In a compound, the sum of the valence units of the atoms of one element must correspond to the total valency of the other.
- In a molecule water N-O-N The valence of H is equal to I, there are 2 such atoms, which means that hydrogen has 2 valence units in total (1×2=2). The valency of oxygen has the same meaning.
- In a compound consisting of two types of atoms, the element located in second place has the lowest valence.
- The valence of the acid residue coincides with the number of H atoms in the acid formula, the valence of the OH group is equal to I.
- In a compound formed by atoms of three elements, the atom that is in the middle of the formula is called the central one. The O atoms are directly bonded to it, and the remaining atoms form bonds with oxygen.
We use these rules to complete tasks.
Concept valence derived from Latin word“valentia” was known back in the middle of the 19th century. The first “extensive” mention of valency was in the works of J. Dalton, who argued that all substances consist of atoms connected to each other in certain proportions. Then, Frankland introduced the very concept of valency, which found further development in the works of Kekule, who spoke about the relationship between valency and chemical bonding, A.M. Butlerov, who in his theory of the structure of organic compounds linked valency with the reactivity of a particular chemical compound and D.I. Mendeleev (in the Periodic Table of Chemical Elements, the highest valency of an element is determined by the group number).
DEFINITION
Valence is the number of covalent bonds that an atom can form when combined with a covalent bond.
The valence of an element is determined by the number of unpaired electrons in an atom, since they take part in the formation of chemical bonds between atoms in the molecules of compounds.
The ground state of an atom (state with minimum energy) is characterized by the electronic configuration of the atom, which corresponds to the position of the element in the Periodic Table. An excited state is a new energy state of an atom, with a new distribution of electrons within the valence level.
Electronic configurations of electrons in an atom can be depicted not only in the form of electronic formulas, but also using electron graphic formulas (energy, quantum cells). Each cell denotes an orbital, an arrow indicates an electron, the direction of the arrow (up or down) indicates the spin of the electron, a free cell represents a free orbital that an electron can occupy when excited. If there are 2 electrons in a cell, such electrons are called paired, if there is 1 electron, they are called unpaired. For example:
6 C 1s 2 2s 2 2p 2
The orbitals are filled as follows: first, one electron with the same spins, and then a second electron with opposite spins. Since the 2p sublevel has three orbitals with the same energy, each of the two electrons occupied one orbital. One orbital remained free.
Determination of the valence of an element using electronic graphic formulas
The valency of an element can be determined by electron-graphical formulas for the electronic configurations of electrons in an atom. Let's consider two atoms - nitrogen and phosphorus.
7 N 1s 2 2s 2 2p 3
Because The valence of an element is determined by the number of unpaired electrons, therefore, the valence of nitrogen is III. Since the nitrogen atom has no empty orbitals, an excited state is not possible for this element. However, III is not the maximum valence of nitrogen, the maximum valence of nitrogen is V and is determined by the group number. Therefore, it should be remembered that using electronic graphic formulas it is not always possible to determine the highest valency, as well as all the valences characteristic of this element.
15 P 1s 2 2s 2 2p 6 3s 2 3p 3
In the ground state, the phosphorus atom has 3 unpaired electrons, therefore, the valence of phosphorus is III. However, in the phosphorus atom there are free d-orbitals, therefore electrons located on the 2s sublevel are able to pair up and occupy vacant orbitals of the d-sublevel, i.e. go into an excited state.
Now the phosphorus atom has 5 unpaired electrons, therefore phosphorus also has a valence of V.
Elements having multiple valence values
Elements of groups IVA – VIIA can have several valency values, and, as a rule, the valency changes in steps of 2 units. This phenomenon is due to the fact that electrons participate in pairs in the formation of a chemical bond.
Unlike the elements of the main subgroups, the elements of the B-subgroups in most compounds do not exhibit a higher valency equal to the group number, for example, copper and gold. In general, transition elements show great diversity chemical properties, which is explained by a large set of valences.
Let us consider the electronic graphic formulas of the elements and establish why the elements have different valences (Fig. 1).
Tasks: determine the valence possibilities of As and Cl atoms in the ground and excited states.
