Different chemical elements differ in their ability to create chemical bonds, that is, to connect with other atoms. Therefore, in complex substances they can only be present in certain proportions. Let's figure out how to determine valency using the periodic table.
There is such a definition of valence: this is the ability of an atom to form a certain number of chemical bonds. Unlike , this quantity is always only positive and is denoted by Roman numerals.
This characteristic for hydrogen is used as a unit, which is taken equal to I. This property shows how many monovalent atoms a given element can combine with. For oxygen, this value is always equal to II.
It is necessary to know this characteristic in order to correctly write chemical formulas of substances and equations. Knowing this value will help establish the relationship between the number of atoms of different types in a molecule.
This concept originated in chemistry in the 19th century. Frankland started a theory explaining the combination of atoms in various proportions, but his ideas about the “binding force” were not very widespread. A vital role in the development of the theory belonged to Kekula. He called the property of forming a certain number of bonds basicity. Kekulé believed that this was a fundamental and unchanging property of every type of atom. Butlerov made important additions to the theory. With the development of this theory, it became possible to visually depict molecules. This was very helpful in studying the structure of various substances.
How can the periodic table help?
You can find valency by looking at the group number in the short-period version. For most elements for which this characteristic is constant (takes only one value), it coincides with the group number.
Such properties have main subgroups. Why? The group number corresponds to the number of electrons in the outer shell. These electrons are called valence electrons. They are responsible for the ability to connect with other atoms.
The group consists of elements with a similar electronic shell structure, and the nuclear charge increases from top to bottom. In short-term form, each group is divided into main and secondary subgroups. Representatives of the main subgroups are s and p elements, representatives of the side subgroups have electrons in d and f orbitals.
How to determine valence chemical elements if it changes? It can coincide with the group number or be equal to the group number minus eight, and also take other values.
Important! The higher and to the right the element, the less its ability to form relationships. The more it is shifted down and to the left, the larger it is.
The way valence changes in the periodic table for a particular type of atom depends on the structure of its electron shell. Sulfur, for example, can be di-, tetra- and hexavalent.
In the ground (unexcited) state of sulfur, two unpaired electrons are located in the 3p sublevel. In this state, it can combine with two hydrogen atoms and form hydrogen sulfide. If sulfur goes into a more excited state, then one electron will move to the free 3d sublevel, and there will be 4 unpaired electrons.
Sulfur will become tetravalent. If you give it even more energy, then another electron will move from the 3s sublevel to 3d. Sulfur will go into an even more excited state and become hexavalent.
Constant and variable
Sometimes the ability to form chemical bonds may change. It depends on which compound the element is included in. For example, sulfur in H2S is divalent, in SO2 it is tetravalent, and in SO3 it is hexavalent. The largest of these values is called the highest, and the smallest is called the lowest. The highest and lowest valencies according to the periodic table can be established as follows: the highest coincides with the group number, and the lowest is equal to 8 minus the group number.
How to determine the valence of chemical elements and whether it changes? We need to establish whether we are dealing with a metal or a non-metal. If it is a metal, you need to establish whether it belongs to the main or secondary subgroup.
- Metals of the main subgroups have a constant ability to form chemical bonds.
- For metals of secondary subgroups - variable.
- For non-metals it is also variable. In most cases, it takes two meanings - higher and lower, but sometimes there can be a greater number of options. Examples are sulfur, chlorine, bromine, iodine, chromium and others.
In compounds, the lowest valence is shown by the element that is higher and to the right in the periodic table, respectively, the highest is the one that is to the left and lower.
Often the ability to form chemical bonds takes on more than two meanings. Then you won’t be able to recognize them from the table, but you will need to learn them. Examples of such substances:
- carbon;
- sulfur;
- chlorine;
- bromine.
How to determine the valence of an element in the formula of a compound? If it is known for other components of the substance, this is not difficult. For example, you need to calculate this property for chlorine in NaCl. Sodium is an element of the main subgroup of the first group, so it is monovalent. Consequently, chlorine in this substance can also create only one bond and is also monovalent.
Important! However, it is not always possible to find out this property for all atoms in a complex substance. Let's take HClO4 as an example. Knowing the properties of hydrogen, we can only establish that ClO4 is a monovalent residue.
How else can you find out this value?
The ability to form a certain number of connections does not always coincide with the group number, and in some cases it will simply have to be learned. Here the table of valency of chemical elements will come to the rescue, which shows the values of this value. The 8th grade chemistry textbook provides values for the ability to combine with other atoms of the most common types of atoms.
| H, F, Li, Na, K | 1 |
| O, Mg, Ca, Ba, Sr, Zn | 2 |
| B, Al | 3 |
| C, Si | 4 |
| Cu | 1, 2 |
| Fe | 2, 3 |
| Cr | 2, 3, 6 |
| S | 2, 4, 6 |
| N | 3, 4 |
| P | 3, 5 |
| Sn, Pb | 2, 4 |
| Cl, Br, I | 1, 3, 5, 7 |
Application
It is worth saying that chemists currently hardly use the concept of valency according to the periodic table. Instead, the concept of oxidation state is used for the ability of a substance to form a certain number of relationships, for substances with structure - covalence, and for substances with an ionic structure - ion charge.
However, the concept under consideration is used for methodological purposes. With its help it is easy to explain why atoms different types combine in the ratios that we observe, and why these ratios are different for different compounds.
On this moment the approach according to which the combination of elements into new substances was always explained using valency according to the periodic table, regardless of the type of bond in the compound, is outdated. Now we know that for ionic, covalent, and metallic bonds there are different mechanisms for combining atoms into molecules.
Useful video
Let's sum it up
Using the periodic table, it is not possible to determine the ability to form chemical bonds for all elements. For those that exhibit one valency according to the periodic table, in most cases it is equal to the group number. If there are two options for this value, then it can be equal to the group number or eight minus the group number. There are also special tables by which you can find out this characteristic.
DEFINITION
Under valency refers to the property of an atom of a given element to attach or replace a certain number of atoms of another element.
A measure of valence can therefore be the number of chemical bonds formed by a given atom with other atoms. Thus, at present, the valence of a chemical element is usually understood as its ability (more in the narrow sense- a measure of its ability) to form chemical bonds (Fig. 1). In the presentation of the valence bond method numeric value valency corresponds to number covalent bonds that an atom forms.
Rice. 1. Schematic formation of water and ammonia molecules.
Table of valency of chemical elements
Initially, the valency of hydrogen was taken as the unit of valency. The valence of another element was expressed by the number of hydrogen atoms that one atom of this element adds to itself or replaces (the so-called hydrogen valency). For example, in compounds of the composition HCl, H 2 O, NH 3, CH 4, the hydrogen valence of chlorine is one, oxygen - two, nitrogen - three, carbon - four.
Then it was decided that the valence of the desired element could also be determined by oxygen, the valence of which, as a rule, is equal to two. In this case, the valence of a chemical element is calculated as twice the number of oxygen atoms that can attach one atom of this element (the so-called oxygen valency). For example, in compounds of the composition N 2 O, CO, SiO 2, SO 3, the oxygen valence of nitrogen is one, carbon - two, silicon - four, sulfur - six.
In fact, it turned out that most chemical elements have different valency values in hydrogen and oxygen compounds: for example, the valency of sulfur in hydrogen is two (H 2 S), and in oxygen - six (SO 3). In addition, most elements exhibit different valencies in their compounds. For example, carbon forms two oxides: CO monoxide and CO 2 dioxide. In the first of which the valency of carbon is II, and in the second - four. It follows that, as a rule, it is impossible to characterize the valency of an element with any one number.
Highest and lowest valencies of chemical elements
The values of the highest and lowest valencies of a chemical element can be determined using the Periodic Table D.I. Mendeleev. The highest valence of an element coincides with the number of the group in which it is located, and the lowest is the difference between the number 8 and the group number. For example, bromine is located in group VIIA, which means its highest valence is VII, and its lowest is I.
There are elements with the so-called. constant valency (metals of groups IA and IIA, aluminum, hydrogen, fluorine, oxygen), which in their compounds exhibit a single oxidation state, which most often coincides with the group number of the Periodic Table D.I. Mendeleev, where they are located).
Elements that are characterized by several valence values (and not always the highest and lowest valence) are called variable-valence. For example, sulfur is characterized by valences II, IV and VI.
To make it easier to remember how many and what valencies are characteristic of a particular chemical element, use tables of valence of chemical elements, which look like this:
Examples of problem solving
EXAMPLE 1
| Exercise | Valence III is characteristic of: a) Ca; b) P; c) O; d)Si? |
| Solution | a) Calcium is a metal. Characterized only possible meaning valency, coinciding with the group number in the Periodic Table D.I. Mendeleev, in which it is located, i.e. The valence of calcium is II. The answer is incorrect. b) Phosphorus is a non-metal. Belongs to a group of chemical elements with variable valence: the highest is determined by the group number in the Periodic Table D.I. Mendeleev, in which it is located, i.e. is equal to V, and the lowest is the difference between the number 8 and the group number, i.e. equal to III. This is the correct answer. |
| Answer | Option (b) |
EXAMPLE 2
| Exercise | Valence III is characteristic of: a) Be; b) F; c) Al; d)C? |
| Solution | In order to give the correct answer to the question posed, we will consider each of the proposed options separately. a) Beryllium is a metal. It is characterized by the only possible valency value, coinciding with the group number in the Periodic Table D.I. Mendeleev, in which it is located, i.e. The valence of beryllium is II. The answer is incorrect. b) Fluorine is a non-metal. It is characterized by the only possible valency value equal to I. The answer is incorrect. c) Aluminum is a metal. It is characterized by the only possible valency value, coinciding with the group number in the Periodic Table D.I. Mendeleev, in which it is located, i.e. The valency of aluminum is III. This is the correct answer. |
| Answer | Option (c) |
Looking at the formulas of various compounds, it is easy to notice that number of atoms of the same element in the molecules of different substances is not identical. 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.
– This is the property of an atom of a given element to attach, retain, or replace a certain number of atoms of another element in chemical reactions. 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 valency 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 valency 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:
MnO
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).
2x = 14,
x = 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.
Instructions
The table is a structure in which chemical elements are arranged according to their principles and laws. That is, we can say that it is a multi-storey “house” in which chemical elements “live”, and each of them has its own apartment under a certain number. “Floors” are located horizontally, which can be small or large. If a period consists of two rows (as indicated by numbering on the side), then such a period is called large. If it has only one row, it is called small.
The table is also divided into “entrances” - groups, of which there are eight in total. Just as in any entrance, apartments are located on the left and right, so here the chemical elements are arranged in the same way. Only in this version their placement is uneven - on one side there are more elements and then they talk about main group, on the other hand, less and this indicates that the group is secondary.
Valency is the ability of elements to form chemical bonds. There is a constant that does not change and a variable that has different meaning depending on what substance the element is part of. When determining valency using the periodic table, you need to pay attention to the following characteristics: the group number of the elements and its type (that is, the main or secondary group). The constant valence in this case is determined by the group number of the main subgroup. To find out the value of the variable valency (if there is one, and usually y), then you need to subtract the number of the group in which the element is located from 8 (a total of 8 - hence the number).
Example No. 1. If you look at the elements of the first group of the main subgroup (alkaline), we can conclude that they all have a valency equal to I (Li, Na, K, Rb, Cs, Fr).
Example No. 2. The elements of the second group of the main subgroup (alkaline earth metals) respectively have valency II (Be, Mg, Ca, Sr, Ba, Ra).
Example No. 3. If we talk about non-metals, then for example, P (phosphorus) is in group V of the main subgroup. Hence its valency will be equal to V. In addition, phosphorus has one more valence value, and to determine it, you must perform step 8 - element number. This means 8 – 5 (group number) = 3. Therefore, the second valence of phosphorus is equal to III.
Example No. 4. Halogens are in group VII of the main subgroup. This means their valence will be VII. However, given that these are non-metals, you need to perform an arithmetic operation: 8 – 7 (element group number) = 1. Therefore, the other valence is equal to I.
For elements of secondary subgroups (and only metals belong to them), the valence must be remembered, especially since in most cases it is equal to I, II, less often III. You will also have to memorize the valencies of chemical elements that have more than two meanings.
Video on the topic
note
Be careful when identifying metals and non-metals. For this purpose, symbols are usually given in the table.
Sources:
- how to correctly pronounce the elements of the periodic table
- what is the valency of phosphorus? X
From school or even earlier, everyone knows that everything around, including ourselves, consists of atoms - the smallest and indivisible particles. Thanks to the ability of atoms to connect with each other, the diversity of our world is enormous. This ability of chemical atoms element form bonds with other atoms is called valency element.
Instructions
For example, you can use two substances– HCl and H2O. This is well known to everyone and water. The first substance contains one hydrogen atom (H) and one chlorine atom (Cl). This suggests that in this compound they form one, that is, they hold one atom near them. Hence, valence both one and the other are equal to 1. It is also easy to determine valence elements that make up a water molecule. It contains two hydrogen and one oxygen atom. Consequently, the oxygen atom formed two bonds to attach two hydrogens, and they, in turn, formed one bond. Means, valence oxygen is 2, and hydrogen is 1.
But sometimes you have to face substances they are more complex in terms of the properties of their constituent atoms. There are two types of elements: constant (hydrogen, etc.) and non-permanent valence Yu. For atoms of the second type, this number depends on the compound they are part of. An example is (S). It can have valences of 2, 4, 6, and sometimes even 8. Determining the ability of elements like sulfur to hold other atoms around it is a little more difficult. To do this you need to know other components substances.
Remember the rule: the product of the number of atoms times valence of one element in the compound must coincide with the same product for the other element. This can be verified by looking again at the water molecule (H2O):
2 (amount of hydrogen) * 1 (its valence) = 2
1 (amount of oxygen) * 2 (its valence) = 2
2 = 2 means everything is defined correctly.
Now test this algorithm on a more complex substance, for example, N2O5 - oxide. It was previously indicated that oxygen has a constant valence 2, so we can compose:
2 (valence oxygen) * 5 (its quantity) = X (unknown valence nitrogen) * 2 (its amount)
By simple arithmetic calculations it can be determined that valence nitrogen in this compound is 5.
Valence is the ability of chemical elements to hold a certain number of atoms of other elements. At the same time, it is the number of bonds formed by a given atom with other atoms. Determining valency is quite simple.
Instructions
Please note that the valency of the atoms of some elements is constant, while others are variable, that is, they tend to change. For example, hydrogen in all compounds is monovalent, since it forms only one. Oxygen is capable of forming two bonds, while being divalent. But y may have II, IV or VI. It all depends on the element with which it is connected. Thus, sulfur is an element with variable valency.
Note that in molecules of hydrogen compounds, calculating the valence is very simple. Hydrogen is always monovalent, and this indicator for the element associated with it will be equal to the number of hydrogen atoms in a given molecule. For example, in CaH2 calcium will be divalent.
Remember the main rule for determining valency: the product of the valence index of an atom of any element and the number of its atoms in any molecule is the product of the valence index of an atom of the second element and the number of its atoms in a given molecule.
Look at the letter formula for this equality: V1 x K1 = V2 x K2, where V is the valency of the atoms of the elements, and K is the number of atoms in the molecule. With its help, it is easy to determine the valence index of any element if the remaining data is known.
Consider the example of the sulfur oxide molecule SO2. Oxygen in all compounds is divalent, therefore, substituting the values into the proportion: Voxygen x Oxygen = Vsulfur x Xers, we get: 2 x 2 = Vsulfur x 2. From here Vsulfur = 4/2 = 2. Thus, the valence of sulfur in this molecule is equal 2.
Video on the topic
Discovery of the periodic law and creation of an ordered system of chemical elements D.I. Mendeleev became the apogee of the development of chemistry in the 19th century. The scientist summarized and systematized extensive knowledge about the properties of elements.
Instructions
In the 19th century there was no idea about the structure of the atom. Discovery by D.I. Mendeleev was only a generalization of experimental facts, but their physical meaning for a long time remained unclear. When the first data appeared on the structure of the nucleus and the distribution of electrons in atoms, it was possible to look at the law and system of elements in a new way. Table D.I. Mendeleev makes it possible to visually trace the properties of the elements found in.
Each element in the table is assigned a specific serial number (H - 1, Li - 2, Be - 3, etc.). This number corresponds to the nucleus (the number of protons in the nucleus) and the number of electrons orbiting the nucleus. The number of protons is thus equal to the number of electrons, which means that under normal conditions the atom is electrically .
The division into seven periods occurs according to the number of energy levels of the atom. Atoms of the first period have a single-level electron shell, the second - a two-level, the third - a three-level, etc. When a new energy level is filled, a new period begins.
The first elements of any period are characterized by atoms that have one electron at the outer level - these are alkali metal atoms. The periods end with atoms of noble gases, which have an external energy level completely filled with electrons: in the first period, noble gases have 2 electrons, in subsequent periods - 8. It is precisely because of the similar structure of the electron shells that groups of elements have similar physics.
In the table D.I. Mendeleev has 8 main subgroups. This number is determined by the maximum possible number of electrons at the energy level.
At the bottom of the periodic table, lanthanides and actinides are distinguished as independent series.
Using the table D.I. Mendeleev, one can observe the periodicity of the following properties of elements: atomic radius, atomic volume; ionization potential; electron affinity forces; electronegativity of the atom; ; physical properties potential connections.
Clearly traceable periodicity of the arrangement of elements in the table D.I. Mendeleev is rationally explained by the sequential nature of filling energy levels with electrons.
Valency is the ability of atoms to attach to themselves a certain number of other atoms.
One atom of another monovalent element is combined with one atom of a monovalent element(HCl) . An atom of a divalent element combines with two atoms of a monovalent element.(H2O) or one divalent atom(CaO) . This means that the valence of an element can be represented as a number that shows how many atoms of a monovalent element an atom of a given element can combine with. The valency of an element is the number of bonds that an atom forms:
Na – monovalent (one bond)
H – monovalent (one bond)
O – divalent (two bonds for each atom)
S – hexavalent (forms six bonds with neighboring atoms)
Rules for determining valency
elements in connections
1. Valence hydrogen mistaken for I(unit). Then, in accordance with the formula of water H 2 O, two hydrogen atoms are attached to one oxygen atom.
2. Oxygen in its compounds always exhibits valence II. Therefore, the carbon in the compound CO 2 (carbon dioxide) has a valence of IV.
3. Higher valence equal to group number .
4. Lowest valence is equal to the difference between the number 8 (the number of groups in the table) and the number of the group in which this element is located, i.e. 8 - N groups .
5. For metals located in “A” subgroups, the valence is equal to the group number.
6. Nonmetals generally exhibit two valences: higher and lower.
For example: sulfur has the highest valency VI and the lowest (8 – 6) equal to II; phosphorus exhibits valences V and III.
7. Valence can be constant or variable.
The valency of elements must be known in order to compose chemical formulas of compounds.
Algorithm for composing the formula of a phosphorus oxide compound
|
Sequencing |
Formulating phosphorus oxide |
|
1. Write the symbols of the elements |
R O |
|
2. Determine the valencies of elements |
V II |
|
3. Find the least common multiple of the numerical values of valences |
5 2 = 10 |
|
4. Find the relationships between atoms of elements by dividing the found smallest multiple by the corresponding valencies of the elements |
10: 5 = 2, 10: 2 = 5; P:O=2:5 |
|
5. Write indices for element symbols |
R 2 O 5 |
|
6. Formula of the compound (oxide) |
R 2 O 5 |
Remember!
Features of compiling chemical formulas of compounds.
1) The lowest valence is shown by the element that is located to the right and above in D.I. Mendeleev’s table, and the highest valence is shown by the element located to the left and below.
For example, in combination with oxygen, sulfur exhibits the highest valency VI, and oxygen the lowest valency II. Thus, the formula for sulfur oxide will be SO 3.
In the compound of silicon with carbon, the first exhibits the highest valency IV, and the second - the lowest IV. So the formula
– SiC. This is silicon carbide, the basis of refractory and abrasive materials.
2) The metal atom comes first in the formula.
2) In the formulas of compounds, the non-metal atom exhibiting the lowest valency always comes in second place, and the name of such a compound ends in “id”.
For example,SaO – calcium oxide, NaCl - sodium chloride, PbS – lead sulfide.
Now you can write the formulas for any compounds of metals and non-metals.