Until now, you have used the chemical formulas of substances given in the textbook, or those that the teacher told you. How to correctly compose chemical formulas?
Chemical formulas of substances are compiled based on knowledge of the qualitative and quantitative composition of the substance. There are a huge number of substances; naturally, it is impossible to remember all the formulas. This is not necessary! It is important to know a certain pattern according to which atoms are able to combine with each other to form new chemical compounds. This ability is called valency.
Valence– the property of atoms of elements to attach a certain number of atoms of other elements. Consider models of molecules of some substances, such as water, methane and carbon dioxide.
It can be seen that in a water molecule, an oxygen atom attaches two hydrogen atoms. Therefore, its valency is two. In a methane molecule, a carbon atom attaches four hydrogen atoms, its valency in this substance is four. The valence of hydrogen in both cases is equal to one.
Carbon exhibits the same valence in carbon dioxide, but unlike methane, the carbon atom attaches two oxygen atoms, since the valency of oxygen is two. There are elements whose valence does not change in compounds. Such elements are said to have constant valence. If the valency of an element can be different, these are elements with variable valence. The valency of some chemical elements is given in Table 2. Valency is usually denoted by Roman numerals. Table 2. Valency of some chemical elements
It is worth noting that the highest valency of an element numerically coincides with the ordinal number of the group of the Periodic System in which it is located. For example, carbon is in group IV, its highest valency is IV. There are three exceptions:
- nitrogen– is in group V, but its highest valence is IV;
- oxygen– is in group VI, but its highest valency is II;
- fluorine– is in group VII, but its highest valence is I.
Based on the fact that all elements are located in eight groups of the Periodic System, valence can take values from I to VIII.
Drawing up formulas of substances using valence
To compile formulas of substances using valence, we will use a certain algorithm:
Determination of valency using the formula of a substance
To determine the valence of elements using the formula of a substance, the reverse procedure is necessary. Let's also consider it using the algorithm:
When studying this section, we considered complex substances that contain only two types of atoms of chemical elements. Formulas for more complex substances are composed differently.
Binary compounds – compounds that contain two types of atoms of elements
To determine the order of the sequence of compounds of atoms, structural (graphical) formulas of substances are used. In such formulas, the valencies of elements are indicated by valence strokes (dashes). For example, a water molecule can be represented as
The graphical formula depicts only the order of connection of atoms, but not the structure of molecules. In space, such molecules may look different. Thus, a water molecule has the angular structural formula:
- Valence– the ability of atoms of elements to attach a certain number of atoms of other chemical elements
- There are elements with constant and variable valence
- The highest valency of a chemical element coincides with its group number in the Periodic Table of Chemical Elements D.I. Mendeleev. Exceptions: nitrogen, oxygen, fluorine
- Binary compounds– compounds that contain two types of atoms of chemical elements
- Graphic formulas reflect the order of bonds of atoms in a molecule using valence strokes
- The structural formula reflects the actual shape of the molecule in space
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 valency 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 valence 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: valency (A) x = valency (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 valence (II); the molecule of this oxide contains five oxygen atoms and two chlorine atoms. Let the valency 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 valency 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 valence of sulfur is six.
Solution. Let the formula of the compound be S x F y . The valence of sulfur is given (VI), the valence 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".
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 the formula H-O-H it is clear that each H atom is connected to only one atom (in this case, oxygen). It follows that its valency 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 valence.
- 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 valence 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 valency 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 valency of the 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 valence 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 valence value for elements for which it is variable is determined by the formula: Group No. in PS - 8. Thus, the lowest valency 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 valency units of the atoms of one element must correspond to the total valence of the other.
- In a water molecule H-O-H, 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 valence of oxygen has the same meaning.
- In a compound consisting of two types of atoms, the element located in second place has the lowest valency.
- 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.
The level of knowledge about the structure of atoms and molecules in the 19th century did not allow us to explain the reason why atoms form a certain number of bonds with other particles. But the ideas of scientists were ahead of their time, and valence is still studied as one of the basic principles of chemistry.
From the history of the emergence of the concept of “valence of chemical elements”
The outstanding English chemist of the 19th century, Edward Frankland, introduced the term “bond” into scientific use to describe the process of interaction of atoms with each other. The scientist noticed that some chemical elements form compounds with the same number of other atoms. For example, nitrogen attaches three hydrogen atoms to an ammonia molecule.
In May 1852, Frankland hypothesized that there was a specific number of chemical bonds that an atom could form with other tiny particles of matter. Frankland used the phrase "cohesive force" to describe what would later be called valence. A British chemist determined how many chemical bonds the atoms of individual elements known in the mid-19th century form. Frankland's work was an important contribution to modern structural chemistry.
Development of views
German chemist F.A. Kekule proved in 1857 that carbon is tetrabasic. In its simplest compound, methane, bonds arise with 4 hydrogen atoms. The scientist used the term “basicity” to denote the property of elements to attach a strictly defined number of other particles. In Russia, the data was systematized by A. M. Butlerov (1861). The theory of chemical bonding received further development thanks to the doctrine of periodic changes in the properties of elements. Its author is another outstanding D.I. Mendeleev. He proved that the valency of chemical elements in compounds and other properties are determined by the position they occupy in the periodic table.
Graphic representation of valency and chemical bond
The ability to visually depict molecules is one of the undoubted advantages of the valency theory. The first models appeared in the 1860s, and since 1864, they have been used to represent circles with a chemical sign inside. Between the symbols of atoms, a dash is indicated, and the number of these lines is equal to the valency value. In those same years, the first ball-and-stick models were produced (see photo on the left). In 1866, Kekulé proposed a stereochemical drawing of the carbon atom in the form of a tetrahedron, which he included in his textbook Organic Chemistry.
The valence of chemical elements and the formation of bonds was studied by G. Lewis, who published his works in 1923. This is the name given to the smallest negatively charged particles that make up the shells of atoms. In his book, Lewis used dots around the four sides to represent valence electrons.
Valence of hydrogen and oxygen
Before its creation, the valence of chemical elements in compounds was usually compared with those atoms for which it was known. Hydrogen and oxygen were chosen as standards. Another chemical element attracted or replaced a certain number of H and O atoms.
In this way, properties were determined in compounds with monovalent hydrogen (the valence of the second element is indicated by a Roman numeral):
- HCl - chlorine (I):
- H 2 O - oxygen (II);
- NH 3 - nitrogen (III);
- CH 4 - carbon (IV).
In the oxides K 2 O, CO, N 2 O 3, SiO 2, SO 3, the oxygen valence of metals and non-metals was determined by doubling the number of added O atoms. The following values were obtained: K (I), C (II), N (III) , Si(IV), S(VI).
How to determine the valence of chemical elements
There are regularities in the formation of chemical bonds involving shared electron pairs:
- The typical valence of hydrogen is I.
- The usual valence of oxygen is II.
- For non-metal elements, the lowest valence can be determined by formula 8 - the number of the group in which they are located in the periodic table. The highest, if possible, is determined by the group number.
- For elements of side subgroups, the maximum possible valency is the same as their group number in the periodic table.
Determination of the valency of chemical elements according to the compound formula is carried out using the following algorithm:
- Write the known value for one of the elements above the chemical symbol. For example, in Mn 2 O 7 the valency of oxygen is II.
- Calculate the total value, for which you need to multiply the valency by the number of atoms of the same chemical element in the molecule: 2 * 7 = 14.
- Determine the valency of the second element for which it is unknown. Divide the value obtained in step 2 by the number of Mn atoms in the molecule.
- 14: 2 = 7. in its higher oxide - VII.
Constant and variable valency
The valency values for hydrogen and oxygen differ. For example, sulfur in the compound H 2 S is divalent, and in the formula SO 3 it is hexavalent. Carbon forms CO monoxide and CO 2 dioxide with oxygen. In the first compound the valence of C is II, and in the second it is IV. The same value in methane CH 4.
Most elements exhibit not a constant, but a variable valency, for example, phosphorus, nitrogen, sulfur. The search for the main causes of this phenomenon led to the emergence of theories of chemical bonds, ideas about the valence shell of electrons, and molecular orbitals. The existence of different values of the same property was explained from the standpoint of the structure of atoms and molecules.
Modern ideas about valency
All atoms consist of a positive nucleus surrounded by negatively charged electrons. The outer shell they form is sometimes unfinished. The completed structure is the most stable, containing 8 electrons (octet). The emergence of a chemical bond due to shared electron pairs leads to an energetically favorable state of atoms.
The rule for forming compounds is to complete the shell by accepting electrons or giving up unpaired ones - depending on which process is easier. If an atom provides negative particles that do not have a pair to form a chemical bond, then it forms as many bonds as it has unpaired electrons. According to modern concepts, the valence of atoms of chemical elements is the ability to form a certain number of covalent bonds. For example, in the hydrogen sulfide molecule H 2 S, sulfur acquires valency II (-), since each atom takes part in the formation of two electron pairs. The "-" sign indicates the attraction of the electron pair to the more electronegative element. For the less electronegative, “+” is added to the valency value.
With the donor-acceptor mechanism, the process involves electron pairs of one element and free valence orbitals of another.
Dependence of valence on atomic structure
Let us consider, using carbon and oxygen as an example, how the valency of chemical elements depends on the structure of the substance. The periodic table gives an idea of the main characteristics of the carbon atom:
- chemical symbol - C;
- element number - 6;
- core charge - +6;
- protons in the nucleus - 6;
- electrons - 6, including 4 external ones, of which 2 form a pair, 2 - unpaired.
If the carbon atom in CO monoxide forms two bonds, then only 6 negative particles come into use. To acquire an octet, the pairs must form 4 external negative particles. Carbon has valency IV (+) in dioxide and IV (-) in methane.
The atomic number of oxygen is 8, the valence shell consists of six electrons, 2 of them do not form pairs and take part in chemical bonds and interactions with other atoms. The typical valence of oxygen is II (-).
Valency and oxidation state
In many cases it is more convenient to use the concept of “oxidation state”. This is the name given to the charge on an atom that it would acquire if all the bonding electrons were transferred to an element that has a higher electronegativity value (EO). The oxidation number in a simple substance is zero. A “-” sign is added to the oxidation state of an element that is more electronegative; a “+” is added to the oxidation state of an element that is less electronegative. For example, for metals of the main subgroups, oxidation states and ion charges equal to the group number with a “+” sign are typical. In most cases, the valency and oxidation state of atoms in the same compound are numerically the same. Only when interacting with more electronegative atoms is the oxidation state positive, with elements with lower EO is negative. The concept of “valency” is often applied only to substances with a molecular structure.
How to determine valency:
Note that the number of oxygen atoms differs in different compounds.
For example, CO 2 and H 2 O. In carbon dioxide there are 1 molecule of carbon and 2 molecules of oxygen, but in water, on the contrary, there are 2 hydrogen and only one oxygen.
The fact is that different substances can attach different numbers of atoms to itself (form a certain number of bonds): hydrogen - 1 atom (1 bond), oxygen - 2 atoms (2 bonds), etc. This property of atoms is called valence(from the Latin “having strength” - the same root as the name “Valentine”, which is also “having strength”).
There is also a certain sequence of this connection, which is expressed in structural formulas, where connections are shown by dashes.
Here is the structural formula for water (H 2 O):
Hydrogen here is bonded only to oxygen, but not to each other. This means that each hydrogen atom has one bond, it is monovalent.
Oxygen has two bonds - it is divalent.
Another structural (graphical) formula for carbon dioxide (CO 2):
Here oxygen is divalent, its atoms are bonded only to tetravalent carbon.
The trivalent nitrogen in ammonia and the tetravalent carbon in methane would look like this:
Valence can also be indicated with Roman numerals on top:
Knowing the valence of one substance, you can easily understand the valency of the second:
For example, Fe 2 O 3 - 3 oxygen atoms have a valency of 2, which means 3*2 = 6, and we have 2 atoms of iron, which means its valence is 6:2 = 3.
However, the valence of some elements may be variable, i.e. differ. depending on the substance with which it comes into contact. Variable valency is indicated in parentheses: CO 2 (IV), CO (II).
In simple substances there is no point in indicating valence. Valence can be of interest only in molecular substances that contain 2 or more elements.
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