Valency is the ability of an atom of a given element to form a certain number of chemical bonds.
Figuratively speaking, valency is the number of “hands” with which an atom clings to other atoms. Naturally, atoms do not have any “hands”; their role is played by the so-called. valence electrons.
You can say it differently: Valence is the ability of an atom of a given element to attach a certain number of other atoms.
The following principles must be clearly understood:
There are elements with constant valence (of which there are relatively few) and elements with variable valence (of which the majority are).
Elements with constant valence must be remembered:
The remaining elements may exhibit different valencies.
The highest valence of an element in most cases coincides with the number of the group in which the element is located.
For example, manganese is in group VII (side subgroup), the highest valence of Mn is seven. Silicon is located in group IV (main subgroup), its highest valency is four.
It should be remembered, however, that the highest valency is not always the only possible one. For example, the highest valency of chlorine is seven (make sure of this!), but compounds in which this element exhibits valences VI, V, IV, III, II, I are known.
It's important to remember a few exceptions: the maximum (and only) valence of fluorine is I (and not VII), oxygen - II (and not VI), nitrogen - IV (the ability of nitrogen to exhibit valency V is a popular myth that is found even in some school textbooks).
Valence and oxidation state are not identical concepts.
These concepts are quite close, but they should not be confused! The oxidation state has a sign (+ or -), the valence does not; the oxidation state of an element in a substance can be zero, the valency is zero only if we are dealing with an isolated atom; the numerical value of the oxidation state may NOT coincide with the valence. For example, the valency of nitrogen in N 2 is III, and the oxidation state = 0. The valence of carbon in formic acid is = IV, and the oxidation state = +2.
If the valence of one of the elements in a binary compound is known, the valency of the other can be found.
This is done very simply. Remember the formal rule: the product of the number of atoms of the first element in a molecule and its valency must be equal to a similar product for the second element.
In the compound A x B y: valence (A) x = valence (B) y
Example 1. Find the valencies of all elements in the compound NH 3.
Solution. We know the valence of hydrogen - it is constant and equal to I. We multiply the valency H by the number of hydrogen atoms in the ammonia molecule: 1 3 = 3. Therefore, for nitrogen, the product of 1 (the number of atoms N) by X (the valence of nitrogen) should also be equal to 3. Obviously, X = 3. Answer: N(III), H(I).
Example 2. Find the valences of all elements in the Cl 2 O 5 molecule.
Solution. Oxygen has a constant valency (II); the molecule of this oxide contains five oxygen atoms and two chlorine atoms. Let the valence of chlorine = X. Let's create the equation: 5 2 = 2 X. Obviously, X = 5. Answer: Cl(V), O(II).
Example 3. Find the valence of chlorine in the SCl 2 molecule if it is known that the valency of sulfur is II.
Solution. If the authors of the problem had not told us the valence of sulfur, it would have been impossible to solve it. Both S and Cl are elements with variable valency. Taking into account additional information, the solution is constructed according to the scheme of examples 1 and 2. Answer: Cl(I).
Knowing the valencies of two elements, you can create a formula for a binary compound.
In examples 1 - 3, we determined valence using the formula; now let’s try to do the reverse procedure.
Example 4. Write a formula for the compound of calcium and hydrogen.
Solution. The valencies of calcium and hydrogen are known - II and I, respectively. Let the formula of the desired compound be Ca x H y. We again compose the well-known equation: 2 x = 1 y. As one of the solutions to this equation, we can take x = 1, y = 2. Answer: CaH 2.
“Why exactly CaH 2? - you ask. - After all, the variants Ca 2 H 4 and Ca 4 H 8 and even Ca 10 H 20 do not contradict our rule!”
The answer is simple: take the minimum possible values of x and y. In the example given, these minimum (natural!) values are exactly 1 and 2.
“So, compounds like N 2 O 4 or C 6 H 6 are impossible?” you ask. “Should these formulas be replaced with NO 2 and CH?”
No, they are possible. Moreover, N 2 O 4 and NO 2 are completely different substances. But the formula CH does not correspond to any real stable substance at all (unlike C 6 H 6).
Despite all that has been said, in most cases you can follow the rule: take the smallest index values.
Example 5. Write a formula for the compound of sulfur and fluorine if it is known that the valency of sulfur is six.
Solution. Let the formula of the compound be S x F y . The valence of sulfur is given (VI), the valency of fluorine is constant (I). We formulate the equation again: 6 x = 1 y. It is easy to understand that the smallest possible values of the variables are 1 and 6. Answer: SF 6.
Here, in fact, are all the main points.
Now check yourself! I suggest you go through a short test on the topic "Valence".
Valence– the ability of elements to attach other elements to themselves.
In simple terms, this is a number that shows how many elements a certain atom can attach to itself.
The key point in chemistry is to correctly write the formulas of compounds.
There are several rules that make it easier for us to correctly compose formulas.
- The valence of all metals of the main subgroups is equal to the group number:
The figure shows an example of the main and secondary subgroups of group I.
2. The valency of oxygen is two
3. The valency of hydrogen is one
4. Non-metals exhibit two types of valence:
- Lowest (8th group)
- Highest (equal to group number)
A) In compounds with metals, non-metals exhibit lower valency!
B) In binary compounds, the sum of the valence of one type of atom is equal to the sum of the valence of another type of atom!
The valency of aluminum is three (aluminum is a group III metal). The valence of oxygen is two. The sum of valence for two aluminum atoms is 6. The sum of valence for three oxygen atoms is also 6.
1) Determine the valences of elements in compounds:
The valency of aluminum is III. In formula 1, atom => total valency is also equal to 3. Therefore, for all chlorine atoms, the valence will also be equal to 3 (rule of binary compounds). 3:3=1. The valence of chlorine is 1.
The valence of oxygen is 2. In a compound there are 3 oxygen atoms => the total valence is 6. For two atoms the total valency is 6 => for one iron atom - 3 (6:2 = 3)
2) Make up formulas for a compound consisting of:
sodium and oxygen
The valency of oxygen is II.
Sodium is a metal of the first group of the main subgroup => its valency is I.
One of the important topics in the study of school is the course regarding valence. This will be discussed in the article.
Valence - what is it?
Valence in chemistry means the property of the atoms of a chemical element to bind atoms of another element to themselves. Translated from Latin - strength. It is expressed in numbers. For example, the valence of hydrogen will always be equal to one. If we take the formula of water - H2O, it can be represented as H - O - H. One oxygen atom was able to bind two hydrogen atoms to itself. This means that the number of bonds that oxygen creates is two. And the valence of this element will be equal to two.
In turn, hydrogen will be divalent. Its atom can be connected to only one atom of a chemical element. In this case with oxygen. More precisely, atoms, depending on the valency of the element, form pairs of electrons. How many such pairs are formed - this will be the valence. The numeric value is called the index. Oxygen has an index of 2.
How to determine the valence of chemical elements using Dmitry Mendeleev’s table
Looking at the periodic table of elements, you will notice vertical rows. They are called groups of elements. Valence also depends on the group. Elements of the first group have the first valency. Second - second. Third - third. And so on.
There are also elements with a constant valency index. For example, hydrogen, halogen group, silver and so on. They definitely need to be learned.
How to determine the valency of chemical elements using formulas?
Sometimes it is difficult to determine valence from the periodic table. Then you need to look at the specific chemical formula. Let's take FeO oxide. Here, iron, like oxygen, will have a valency index of two. But in Fe2O3 oxide it’s different. Iron will be ferric.
You must always remember the different ways to determine valence and not forget them. Know its constant numerical values. Which elements have them? And, of course, use the table of chemical elements. And also study individual chemical formulas. It is better to present them in schematic form: H – O – H, for example. Then the connections are visible. And the number of dashes (dashes) will be the numerical value of the valence.
In chemistry lessons, you have already become acquainted with the concept of valence of chemical elements. We have collected all useful information on this issue in one place. 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.
Through chemical analysis, scientists of the past determined the composition of a chemical compound: how many atoms of various 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 the appearance of such compounds.
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 the covalent bonds in substances more visual and understandable for you, write their 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 a 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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It is difficult to overestimate the role of iron for the human body, because it is what contributes to the “creation” of blood, its content affects the level of hemoglobin and myoglobin, iron normalizes the functioning of the enzyme system. But what is this element from a chemical point of view? What is the valence of iron? This will be discussed in this article.
A little history
Humanity knew about this chemical element and even owned products made from it back in the 4th century BC. These were the peoples of Ancient Egypt and Sumer. It was they who first began to make jewelry and weapons from an alloy of iron and nickel, which were found during archaeological excavations and carefully studied by chemists.
A little later, the Aryan tribes that moved to Asia learned to extract solid iron from ore. It was so valuable to the people of that time that the products were plated with gold!
Characteristics of iron
Iron (Fe) ranks fourth in terms of its content in the depths of the earth's crust. It occupies a place in group 7 of period 4 and is number 26 in the periodic chemical table of elements. The valence of iron is directly dependent on its position in the table. But more on that later.
This metal is most common in nature in the form of ore, found in water as a mineral, and also in various compounds.
The largest amount of iron reserves in the form of ore is located in Russia, Australia, Ukraine, Brazil, USA, India, and Canada.
Physical properties
Before moving on to the valency of iron, it is necessary to take a closer look at its physical properties, so to speak, to take a closer look at it.
This metal is quite ductile, but is capable of increasing hardness through its interaction with other elements (for example, carbon). It also has magnetic properties.
In a humid environment, iron can corrode, that is, rust. Although absolutely pure metal is more resistant to moisture, if it contains impurities, they provoke corrosion.
Iron interacts well with acidic environments and can even form salts of ferric acid (provided there is a strong oxidizing agent).
In the air, it quickly becomes covered with an oxide film, which protects it from interactions.
Chemical properties
This element also has a number of chemical properties. Iron, like the rest of the elements of the periodic table, has a charge on the atomic nucleus, which corresponds to the atomic number +26. And there are 26 electrons rotating near the nucleus.
In general, if we consider the properties of iron - a chemical element, then it is a metal with a low active ability.
Interacting with weaker oxidizing agents, iron forms compounds where it is divalent (that is, its oxidation state is +2). And if with strong oxidizing agents, then the oxidation state of iron reaches +3 (that is, its valence becomes equal to 3).
When interacting with chemical elements that are not metals, Fe acts as a reducing agent towards them, and its oxidation state becomes, in addition to +2 and +3, even +4, +5, +6. Such compounds have very strong oxidizing properties.
As noted above, iron in the air becomes covered with an oxide film. And when heated, the reaction rate increases and iron oxide with valence 2 (temperature less than 570 degrees Celsius) or oxide with valency 3 (temperature more than 570 degrees Celsius) can be formed.
The interaction of Fe with halogens leads to the formation of salts. The elements fluorine and chlorine oxidize it to +3. Bromine is up to +2 or +3 (it all depends on the conditions for the chemical transformation when interacting with iron).
When interacting with iodine, the element is oxidized to +2.
By heating iron and sulfur, iron sulfide with valency 2 is obtained.
If ferrum is melted and combined with carbon, phosphorus, silicon, boron, nitrogen, you get compounds called alloys.
Iron is a metal, so it also interacts with acids (this was also briefly discussed above). For example, sulfuric and nitric acids, which have a high concentration, do not affect iron in a low-temperature environment. But as soon as it rises, a reaction occurs, as a result of which iron is oxidized to +3.
The higher the acid concentration, the higher the temperature must be given.
By heating divalent iron in water, we obtain its oxide and hydrogen.
Fe also has the ability to displace metals that have reduced activity from aqueous solutions of salts. At the same time, it is oxidized to +2.
As the temperature rises, iron reduces metals from oxides.
What is valence
Already in the previous section, the concept of valency, as well as oxidation state, was encountered a little. It's time to consider the valence of iron.
But first you need to understand what kind of property of chemical elements this is.
Chemicals are almost always constant in their composition. For example, in the formula of water H2O there is 1 oxygen atom and 2 hydrogen atoms. The same is true with other compounds that involve two chemical elements, one of which is hydrogen: 1-4 hydrogen atoms can be added to 1 atom of a chemical element. But not the other way around! Therefore, it is clear that hydrogen attaches to itself only 1 atom of another substance. And it is this phenomenon that is called valency - the ability of atoms of a chemical element to attach a specific number of atoms of other elements.
Valency value and graphical formula
There are elements of the periodic table that have a constant valence - these are oxygen and hydrogen.
And there are chemical elements in which it changes. For example, iron is often 2- and 3-valent, sulfur is 2, 4, 6, carbon is 2 and 4. These are elements with variable valency.
Also, knowing the valency of one of the elements in a compound, you can determine the valency of the other.
Valency of iron
As noted, iron is an element with variable valence. And it can fluctuate not only between indicators 2 and 3, but also reach 4, 5 and even 6.
Of course, he studies the valence of iron in more detail. Let us briefly consider this mechanism at the level of the simplest particles.
Iron is a d-element, which includes 31 more elements of the periodic table (these are periods 4-7). With increasing serial number, the properties of d-elements acquire slight changes. The atomic radius of these substances also increases slowly. They have a variable valence, which depends on the fact that the outer d-electron sublevel is incomplete.
Therefore, for iron, valence electrons are not only c-electrons located in the outer layer, but also unpaired 3D electrons of the outer layer. And, as a result, the valence of Fe in chemical compounds can be equal to 2, 3, 4, 5, 6. Basically, it is equal to 2 and 3 - these are more stable with other substances. In less stable ones, it exhibits a valency of 4, 5, 6. But such compounds are less common.
Divalent ferrum
When 2-valent iron reacts with water, iron oxide (2) is obtained. This compound is black in color. It interacts quite easily with hydrochloric (low concentration) and nitric (high concentration) acids.
If such an oxide of 2-valent iron reacts either with hydrogen (temperature 350 degrees Celsius) or with carbon (coke) at 1000 degrees, then it is restored to a pure state.
Divalent iron oxide is extracted using the following methods:
- through the connection of oxide of 3-valent iron with carbon monoxide;
- when heating pure Fe, with low oxygen pressure;
- when decomposing ferrous oxalate in a vacuum environment;
- when pure iron interacts with its oxides, the temperature is 900-1000 degrees Celsius.
As for the natural environment, divalent iron oxide is present in the form of the mineral wustite.
There is also a way to determine the valence of iron in a solution - in this case, it has an indicator of 2. It is necessary to carry out reactions with red salt (potassium hexacyanoferrate) and with alkali. In the first case, a dark blue precipitate is obtained - a complex salt of divalent iron. In the second - obtaining a dark gray-green precipitate - iron hydroxide, also 2-valent, while 3-valent iron hydroxide has a dark brown color in solution.
Ferric iron
Trivalent ferrum oxide has a powdery structure, the color of which is red-brown. It also has names: iron oxide, red pigment, food coloring, crocus.
In nature, this substance occurs in the form of a mineral - hematite.
The oxide of such iron no longer interacts with water. But it combines with acids and alkalis.
Iron oxide (3) is used to color materials used in construction:
- bricks;
- cement;
- ceramic products;
- concrete;
- paving slabs;
- floor coverings (linoleum).
Iron in the human body
As noted at the beginning of the article, the substance iron is an important component of the human body.
When this element is not enough, the following consequences may occur:
- increased fatigue and sensitivity to cold;
- dry skin;
- decreased brain activity;
- deterioration of the strength of the nail plate;
- dizziness;
- digestive problems;
- gray hair and hair loss.
Iron accumulates, as a rule, in the spleen and liver, as well as the kidneys and pancreas.
A person's diet should include foods containing iron:
- beef liver;
- buckwheat porridge;
- peanut;
- pistachios;
- canned green peas;
- dried porcini mushrooms;
- chicken eggs;
- spinach;
- dogwood;
- apples;
- pears;
- peaches;
- beet;
- seafood.
Lack of iron in the blood leads to a decrease in hemoglobin and the development of a disease such as iron deficiency anemia.