- monovalent hydrogen, halogens, alkali metals
- divalent oxygen, alkaline earth metals.
- trivalent aluminum (Al) and boron (B).
- the highest valence corresponds (equals) to the group number;
- the lowest valence is determined by the formula: the group number is 8.
- II in H2S compound
- IV in SO2 compound
- VI in SO3 compound
In order to determine the valency of a substance, you need to look at the periodic table chemical elements Mendeleev, the designations in Roman numerals will be the valencies of certain substances in this table. For example, HO, hydrogen (H) will always be monovalent a, and oxygen (O) will always be divalent. Below is a cheat sheet that I hope will help you)
First of all, it is worth noting that chemical elements can have both constant and variable valency. As for the constant valence, then you simply need to memorize such elements
Alkali metals, hydrogen, and halogens are considered monovalent;
But trivalent boron and aluminum.
So, now let's go through the periodic table to determine the valency. The highest valence for an element is always equated to its group number
The lower valency is found out by subtracting the group number from 8. Non-metals are endowed with lower valency to a greater extent.
Chemical elements can be of constant or variable valency. Elements with constant valence needs to be learned. Always
Valence can be determined from the periodic table. The highest valence of an element is always equal to the number of the group in which it is located.
Inferior variable valence most often non-metals. To find out lower valence, the group number is subtracted from 8 - the result will be the desired value. For example, sulfur is in group 6 and its highest valence is VI, the lowest valence will be II (86 = 2).
According to the school definition, valency is the ability of a chemical element to form one or another number of chemical bonds with other atoms.
As you know, valence is constant (when a chemical element always forms the same number of bonds with other atoms) and variable (when, depending on a particular substance, the valency of the same element changes).
The periodic system of chemical elements of D. I. Mendeleev will help us determine the valency.
The following rules apply:
1) Maximum the valency of a chemical element is equal to the group number. For example, chlorine is in the 7th group, which means that its maximum valency is 7. Sulfur: it is in the 6th group, which means that it does not have a maximum valency of 6.
2) Minimum valence for non-metals equals 8 minus the group number. For example, the minimum valency of the same chlorine is 8 7, that is, 1.
Alas, there are exceptions to both rules.
For example, copper is in the 1st group, however, the maximum valence of copper is not 1, but 2.
Oxygen is in the 6th group, but its valence is almost always 2, and not at all 6.
It is useful to remember the following rules:
3) All alkaline metals (metals of group I, main subgroup) always have valency 1. For example, the valency of sodium is always 1 because it is an alkali metal.
4) All alkaline earth metals (metals of group II, main subgroup) always have valency 2. For example, the valency of magnesium is always 2 because it is an alkaline earth metal.
5) Aluminum always has a valence of 3.
6) Hydrogen always has a valence of 1.
7) Oxygen almost always has a valence of 2.
8) Carbon almost always has a valence of 4.
It should be remembered that in different sources the definitions of valency may differ.
More or less precisely, valence can be defined as the number of shared electron pairs by which a given atom is connected to others.
According to this definition, the valency of nitrogen in HNO3 is 4, not 5. Nitrogen cannot be pentavalent, because in this case, 10 electrons would circle around the nitrogen atom. And this cannot be, because the maximum of electrons is 8.
The valence of any chemical element is its property, or rather the property of its atoms (atoms of this element) to hold a certain number of atoms, but of another chemical element.
There are chemical elements with both constant and variable valency, which varies depending on which element it (the given element) is in conjunction with or enters.
Valencies of some chemical elements:
Now let's move on to how the valence of an element is determined from the table.
So, valency can be determined by periodic table:
From the school course in chemistry, we know that all chemical elements can be with constant or variable valency. Elements with a constant valency just need to be remembered (for example, hydrogen, oxygen, alkali metals and other elements). Valency is easy to determine from the periodic table, which is in any chemistry textbook. The highest valence corresponds to its number of the group in which it is located.
The valency of any element can be determined by the periodic table itself, by the group number.
At least, this can be done in the case of metals, because their valence is equal to the group number.
With non-metals, a slightly different story: their highest valency (in compounds with oxygen) is also equal to the group number, but the lower valency (in compounds with hydrogen and metals) must be determined by the following formula: 8 - group number.
The more you work with chemical elements, the better you remember their valency. And for starters, this cheat sheet is enough:
Those elements whose valency is not constant are highlighted in pink.
Valetity is the ability of atoms of some chemical elements to attach atoms of other elements to themselves. For successful writing formulas, right decision tasks, you need to know well how to determine valency. First you need to learn all the elements with constant valency. Here they are: 1. Hydrogen, halogens, alkali metals (always monovalent); 2. Oxygen and alkaline earth metals (bivalent); 3. B and Al (trivalent). To determine the valence according to the periodic table, you need to find out in which group the chemical element is located and determine whether it is in the main group or side.
An element can have one or more valences.
The maximum valence of elements is equal to the number of valence electrons. We can determine valence by knowing the location of the element in the periodic table. The maximum valency number is equal to the number of the group in which the required element is located.
Valence is indicated by a Roman numeral and is usually written in the upper right corner of the element symbol.
Some elements may have different valencies in different compounds.
For example, sulfur has the following valencies:
The rules for determining valence are not as easy to use, so they need to be remembered.
It is easy to determine the valence according to the periodic table. As a rule, it corresponds to the number of the group in which the element is located. But there are elements that in different compounds can have different valencies. In this case, we are talking about constant and variable valency. The variable can be maximum, equal to the group number, or it can be minimum or intermediate.
But it is much more interesting to determine the valency in compounds. There are a number of rules for this. First of all, it is easy to determine the valency of the elements if one element in the compound has a constant valency, for example, it is oxygen or hydrogen. On the left is a reducing agent, that is, an element with a positive valency, on the right is an oxidizing agent, that is, an element with a negative valence. The index of an element with constant valency is multiplied by that valence and divided by the index of an element with unknown valency.
Example: silicon oxides. The valency of oxygen is -2. Find the valency of silicon.
SiO 1*2/1=2 The valency of silicon in monoxide is +2.
SiO2 2*2/1=4 The valency of silicon in dioxide is +4.
When considering the chemical elements, one can notice that the number of atoms in the same element in different substances varies. How to write down the formula correctly and not make a mistake in the index of a chemical element? This is easy to do if you have an idea of \u200b\u200bwhat valence is.
What is valence for?
The valence of chemical elements is the ability of the atoms of an element to form chemical bonds, that is, to attach other atoms to themselves. A quantitative measure of valency is the number of bonds that a given atom forms with other atoms or atomic groups.
Currently, valence is the number of covalent bonds (including those that have arisen by the donor-acceptor mechanism) by which a given atom is connected to others. This does not take into account the polarity of the bonds, which means that the valence has no sign and cannot be equal to zero.
A covalent chemical bond is a bond carried out through the formation of common (bonding) electron pairs. If there is one common electron pair between two atoms, then such a bond is called single, if two - double, if three - triple.
How to find valence?
The first question that worries 8th grade students who have begun to study chemistry is how to determine the valency of chemical elements? The valency of a chemical element can be viewed in a special table of the valency of chemical elements
Rice. 1. Table of valency of chemical elements
The valency of hydrogen is taken as unity, since a hydrogen atom can form one bond with other atoms. The valency of other elements is expressed by a number that shows how many hydrogen atoms an atom of a given element can attach to itself. For example, the valence of chlorine in a molecule of hydrogen chloride is equal to one. Therefore, the formula for hydrogen chloride will look like this: HCl. Since both chlorine and hydrogen have a valency of one, no index is used. Both chlorine and hydrogen are monovalent, since one hydrogen atom corresponds to one chlorine atom.
Consider another example: the valence of carbon in methane is four, the valence of hydrogen is always one. Therefore, the index 4 should be placed next to hydrogen. Thus, the formula for methane looks like this: CH 4.
Many elements form compounds with oxygen. Oxygen is always divalent. Therefore, in the formula of water H 2 O, where monovalent hydrogen and divalent oxygen always occur, the index 2 is placed next to hydrogen. This means that the water molecule consists of two hydrogen atoms and one oxygen atom.
Rice. 2. Graphic formula of water
Not all chemical elements have a constant valency, for some it may vary depending on the compounds where this element is used. Elements with constant valency include hydrogen and oxygen, elements with variable valence include, for example, iron, sulfur, carbon.
How to determine valence by the formula?
If you don’t have a valence table before your eyes, but there is a formula for a chemical compound, then it is possible to determine the valence by the formula. Take for example the formula manganese oxide - Mn 2 O 7
Rice. 3. Manganese oxide
As you know, oxygen is divalent. To find out what valency manganese has, it is necessary to multiply the oxygen valence by the number of gas atoms in this compound:
The resulting number is divided by the number of manganese atoms in the compound. It turns out:
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From the materials of the lesson, you will learn that the constancy of the composition of a substance is explained by the presence of certain valence possibilities in the atoms of chemical elements; get acquainted with the concept of "valency of atoms of chemical elements"; learn to determine the valence of an element by the formula of a substance, if the valency of another element is known.
Topic: Initial chemical ideas
Lesson: Valency of chemical elements
The composition of most substances is constant. For example, a water molecule always contains 2 hydrogen atoms and 1 oxygen atom - H 2 O. The question arises: why do substances have a constant composition?
Let's analyze the composition of the proposed substances: H 2 O, NaH, NH 3, CH 4, HCl. All of them consist of atoms of two chemical elements, one of which is hydrogen. There can be 1,2,3,4 hydrogen atoms per atom of a chemical element. But no substance will per hydrogen atom have to a few atoms of another chemical element. Thus, a hydrogen atom can attach to itself the minimum number of atoms of another element, or rather, only one.
The property of the atoms of a chemical element to attach to itself a certain number of atoms of other elements is called valency.
Some chemical elements have constant valence values (for example, hydrogen (I) and oxygen (II)), others may show several valence values (for example, iron (II, III), sulfur (II, IV, VI), carbon (II, IV)), they are called elements with variable valency. The valency values of some chemical elements are given in the textbook.
Knowing the valencies of chemical elements, it is possible to explain why a substance has just such a chemical formula. For example, the formula for water is H 2 O. Let's denote the valence capabilities of a chemical element with dashes. Hydrogen has valence I, and oxygen has II: H- and -O-. Each atom can fully use its valence capabilities if there are two hydrogen atoms per oxygen atom. The sequence of connecting atoms in a water molecule can be represented as a formula: H-O-H.
The formula that shows the sequence of connection of atoms in a molecule is called graphic(or structural).
Rice. 1. Graphic formula of water
Knowing the formula of a substance consisting of atoms of two chemical elements, and the valency of one of them, it is possible to determine the valence of another element.
Example 1 Let's determine the valence of carbon in the substance CH 4. Knowing that the valence of hydrogen is always equal to I, and carbon has attached 4 hydrogen atoms to itself, it can be argued that the valence of carbon is equal to IV. The valence of atoms is indicated by a Roman numeral over the sign of the element: .
Example 2 Let us determine the valence of phosphorus in the compound P 2 O 5. To do this, you must perform the following steps:
1. above the sign of oxygen, write down the value of its valency - II (oxygen has a constant valency value);
2. multiplying the oxygen valence by the number of oxygen atoms in the molecule, find total number units of valence - 2 5=10;
3. Divide the resulting total number of valency units by the number of phosphorus atoms in the molecule - 10:2=5.
Thus, the valence of phosphorus in this compound is equal to V -.
1. Emelyanova E.O., Iodko A.G. Organization cognitive activity students in chemistry classes in grades 8-9. Supporting notes with practical tasks, tests: Part I. - M .: School Press, 2002. (p. 33)
2. Ushakova O.V. Chemistry workbook: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. Grade 8” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006. (p. 36-38)
3. Chemistry: 8th grade: textbook. for general institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§16)
4. Chemistry: inorg. chemistry: textbook. for 8 cells. general education institutions / G.E. Rudzitis, F.G. Feldman. - M .: Education, Moscow Textbooks OJSC, 2009. (§§11,12)
5. Encyclopedia for children. Volume 17. Chemistry / Chapter. edited by V.A. Volodin, leading. scientific ed. I. Leenson. – M.: Avanta+, 2003.
Additional web resources
1. A single collection of digital educational resources ().
2. Electronic version of the journal "Chemistry and Life" ().
Homework
1. p.84 No. 2 from the textbook "Chemistry: 8th grade" (P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M .: AST: Astrel, 2005).
2. With. 37-38 №№ 2,4,5,6 from Workbook in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. Grade 8” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006.
Considering the formulas of various compounds, it is easy to see 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 typical not only for hydrogen.
How to guess which index to put next to the designation of a chemical element? How are the formulas of a substance formed? 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, retain, or replace in chemical reactions a certain number of atoms of another element. The unit of valency is the valency of the 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 univalent 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 bivalent.
Permanent valence:
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 valency of the required element is determined by the valency 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 will not be difficult to find the valence of elements in the following compounds: Na 2 O (valence Na – 1,O – 2), Al 2 O 3 (Al – 3,O – 2).
The chemical formula of a given substance can be compiled only by knowing the valency of the elements. For example, it is easy to formulate 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 valencies 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 valency 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 valency of Mn in a compound is 7, and O – 2, then the compound formula will look like this Mn 2 O 7.
How did we get the formula?
Consider the algorithm for compiling formulas by valency for those consisting of two chemical elements.
There is a rule that the number of valences in one chemical element is equal to the number of valences in another. 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 next the symbols of chemical elements:
2.
We put over the chemical elements the numbers of their valence (the valency of a chemical element can be found in the periodic table of Mendelev, for manganese –
7, have oxygen –
2.
3. Find the least common multiple ( smallest number, which is evenly divisible by 7 and 2). This number is 14. We divide it by the valencies of the elements 14: 7 \u003d 2, 14: 2 \u003d 7, 2 and 7 will be indices, respectively, for phosphorus and oxygen. We substitute indexes.
Knowing the valence of one chemical element, following the rule: the valency of one element × the number of its atoms in a molecule = the valency of another element × the number of atoms of this (another) element, one can determine the valence of another.
Mn 2 O 7 (7 2 = 2 7).
The concept of valency was introduced into chemistry before the structure of the atom was known. It has now been established that this property of an element is related to the number of outer electrons. For many elements, the maximum valence results from the position of those elements in the periodic table.
Do you have any questions? Want to know more about valency?
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So far you have enjoyed chemical formulas substances given in the textbook, or those that the teacher called you. How to write chemical formulas correctly?
Chemical formulas of substances are compiled on the basis of knowledge of the qualitative and quantitative composition of the substance. There are a huge number of substances, it is naturally 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 ones. chemical compounds. This ability is called valency.
Valence- the property of the atoms of elements to attach a certain number of atoms of other elements
Let's 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 the methane molecule, the 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 valency in carbon dioxide, but unlike methane, the carbon atom attaches two oxygen atoms, since the valency of oxygen is two.
There are elements whose valency does not change in compounds. Such elements are said to have permanent valency. If the valence of an element can be different, these are elements with variable valency. The valency of some chemical elements is shown in Table 2. Valency is usually denoted by Roman numbers.
Table 2. Valency of some chemical elements
| Element symbol | Valence | Element symbol | Valence |
| H, Li, Na, K, F, Ag | I | C, Si, Sn, Pb | II, IV |
| Be, Mg, Ca, Ba, Zn, O | II | N | I, II, III, IV |
| Al, B | III | P, As, Sb | III, V |
| S | II, IV, VI | Cl | I, II, III, IV, V, VII |
| Br, I | I, III, V | Ti | II, III, IV |
It is worth noting that the highest valence of an element numerically coincides with the serial 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 valency is IV;
- oxygen- is in group VI, but its highest valency is II;
- fluorine- is in group VII, but its highest valency is I.
Based on the fact that all elements are located in eight groups of the Periodic System, valency can take on the values from I to VIII.
Compilation of formulas of substances using valency
To draw up formulas of substances using valency, we use a certain algorithm:
Determination of valency according to the formula of a substance
To determine the valence of elements according to the formula of a substance, the reverse procedure is necessary. Consider it also using the algorithm:
When studying this paragraph, complex substances were considered, which include only two types of atoms of chemical elements. Formulas for more complex substances are compiled differently.
Binary connections - compounds that contain two types of atoms of elements
To determine the order of the sequence of connecting atoms, structural (graphic) formulas of substances are used. In such formulas, the valency of elements is indicated by valence strokes (dashes). For example, a water molecule can be represented as
N─O─N
The graphic formula depicts only the order of connection of atoms, but not the structure of molecules. In space, such molecules may look different. So, the water molecule has an 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 valency
- The highest valency of a chemical element coincides with its group number in the Periodic Table of chemical elements of D.I. Mendeleev. Exceptions: nitrogen, oxygen, fluorine
- Binary connections- 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 real shape of the molecule in space
