Chemists around the world reflect the composition of simple and complex substances very beautifully and concisely in the form of chemical formulas. Chemical formulas are analogues of words that are written using letters - signs of chemical elements.
Let's use chemical symbols to express the composition of the most common substance on Earth - water. A water molecule contains two hydrogen atoms and one oxygen atom. Now let's translate this sentence into a chemical formula using the chemical symbols (hydrogen - H and oxygen - O). We write the number of atoms in the formula using indices - the numbers below to the right of the chemical symbol (the index 1 for oxygen is not written): H 2 0 (read "ash-two-o").
The formulas of simple substances of hydrogen and oxygen, the molecules of which consist of two identical atoms, are written as follows: H 2 (read "ash-two") and 0 2 (read "o-two") (Fig. 26).
Rice. 26.
Models of molecules and formulas of oxygen, hydrogen and water
To reflect the number of molecules, coefficients are used that are written in front of chemical formulas: for example, the entry 2CO 2 (read "two-ce-o-two") means two carbon dioxide molecules, each of which consists of one carbon atom and two oxygen atoms.
Coefficients are written similarly when the number of free atoms of a chemical element is indicated. For example, we need to write down the expression: five iron atoms and seven oxygen atoms. Do it this way: 5Fe and 7O.
The sizes of molecules, and even more so of atoms, are so small that they cannot be seen even in the best optical microscopes, giving an increase of 5-6 thousand times. They can not be seen in electron microscopes, giving an increase of 40 thousand times. Naturally, the negligibly small size of molecules and atoms corresponds to their negligible masses. Scientists have calculated, for example, that the mass of a hydrogen atom is 0.000 000 000 000 000 000 000 001 674 g, which can be represented as 1.674 10 -24 g, the mass of the oxygen atom is 0.000 000 000 000 000 000 000 026 667 g, or 2, 6667 10 -23 g, the mass of a carbon atom is 1.993 10 -23 g, and the mass of a water molecule is 3.002 10 -23 g.
Let's calculate how many times the mass of an oxygen atom is greater than the mass of a hydrogen atom, the lightest element:
Similarly, the mass of a carbon atom is 12 times greater than the mass of a hydrogen atom:
Rice. 27. The mass of a carbon atom is equal to the mass of 12 hydrogen atoms
The mass of a water molecule is 18 times greater than the mass of a hydrogen atom (Fig. 28). These values show how many times the mass of an atom of a given chemical element is greater than the mass of a hydrogen atom, i.e., they are relative.
Rice. 27. The mass of an atom of water is equal to the mass of 18 hydrogen atoms
Currently, physicists and chemists are of the opinion that the relative atomic mass of an element is a value showing how many times the mass of its atom is greater than 1/12 of the mass of a carbon atom. Relative atomic mass is denoted Ar, where r is the initial letter of the English word relative, which means "relative". For example, A r (0) = 16, A r (C) = 12, A r (H) = 1.
Each chemical element has its own value of relative atomic mass (Fig. 29). The values of the relative atomic masses of chemical elements are indicated in the cells corresponding to them in the table of D. I. Mendeleev.
Rice. 29.
Each element has its own relative atomic mass.
Similarly, the relative molecular weight of a substance is denoted by M r, for example, M r (H 2 0) \u003d 18.
The relative atomic mass of the element A r and the relative molecular mass of the substance M r are quantities that do not have units of measurement.
To find out the relative molecular mass of a substance, it is not necessary to divide the mass of its molecule by the mass of the hydrogen atom. You just need to add the relative atomic masses of the elements that form the substance, taking into account the number of atoms, for example:
The chemical formula contains important information about the substance. For example, the formula C0 2 shows the following information:
Let us calculate the mass fractions of the elements carbon and oxygen in carbon dioxide CO 2 .
Keywords and phrases
- Chemical formula.
- Indices and coefficients.
- Relative atomic mass (A r).
- Relative molecular weight (M r).
- Mass fraction of an element in a substance.
Work with computer
- Refer to the electronic application. Study the material of the lesson and complete the suggested tasks.
- Search the Internet for email addresses that can serve as additional sources that reveal the content of the keywords and phrases of the paragraph. Offer the teacher your help in preparing a new lesson - make a report on the key words and phrases of the next paragraph.
Questions and tasks
- What do the entries mean: 3H; 2H 2 O; 5O2?
- Write down the formula of sucrose if it is known that its molecule contains twelve carbon atoms, twenty-two hydrogen atoms and eleven oxygen atoms.
- Using Figure 2, write down the formulas of the substances and calculate their relative molecular weights.
- Which form of existence of the chemical element oxygen corresponds to each of the following entries: 3O; 5O2; 4CO 2 ?
- Why do the relative atomic mass of an element and the relative molecular mass of a substance have no units of measurement?
- In which of the substances whose formulas are SO 2 and SO 3, the mass fraction of sulfur is greater? Support your answer with calculations.
- Calculate the mass fractions of elements in nitric acid HNO 3 .
- Give a complete characterization of glucose C 6 H 12 0 6 using the example of describing carbon dioxide CO 2.
1. how much calcium oxide substance has a mass of 140 g? (CaO)
2. Calculate the mass and volume of carbon dioxide CO2 by the amount of substance 2.2 mol.
Answer the questions.
3. Position of metals in PSCE, features of their electronic structure. General physical properties of metal. Write the electronic formulas for calcium, lithium and aluminum.
Answer the questions.Yes/No
1. Do bases necessarily include a hydroxy group??? Not really
2. Water-soluble hydroxides are called alkalis??? Not really
3. The number of hydroxy groups per atom of a metal element is twice the valence of this element ??? Not really
4. Alkalis are part of almost all the minerals of the Earth??? Not really
5. In the presence of alkalis, phenolphthalein becomes colorless??? Not really
6. Sodium oxide reacts with water to form alkali?? Not really
7. Alkalis can be obtained through the interaction of the most active metals with water ??? Not really
8. Cuprum(2) oxide actively interacts with water? Not really
9. Is aluminum hydroxide an alkali? Not really
10. Does methyl orange turn red in the presence of alkalis? Not really
11. Is zinc hydroxide an amphoteric hydroxide? Not really
12. Barium hydroxide has the formula Ba(OH)3? Not really
Before answering a question, be sure to write the number of the question you are answering. Thank you very much in advance!
Give the characteristics of the chemical element C according to the plan:a) position in the PSCE - period, group, number of energy levels, number of electrons in the last level;
b) the formula of the higher oxide, its nature;
c) the formula of the higher hydroxide, its nature;
d) formula and name of the volatile hydrogen compound.
1) NO
2) NaCl
3) H2
4) H2O
In the list of substances, acid oxides include:
1) Na2O, N2O5, CaO
2) N2O5, SO2, SiO2
3) CaO, SiO2, SO3
4) CuO, CO2, MgO
Which of the following substances will interact with nitric oxide (V).
1) KOH;
2) HCl;
3) NaCl;
4) O2
What are the chemical properties of calcium oxide?
Choose the correct answer.
1) gas under normal conditions;
2) the substance is solid, white;
3) black solid;
4) colorless volatile liquid.
A5. What is the name of SO3?
1) sulfur oxide;
2) sulfur oxide (II)
3) sulfur oxide (VI)
4) sulfur oxide (IV).
A6. Oxides are formed:
1) when burning simple and complex substances;
2) during the decomposition of soluble bases;
3) only during the combustion of complex substances;
4) during the decomposition of silicic acid.
Which of the following substances will react with zinc oxide?
1) H2O
2) HCl
3) O2
4) Al2O3
Establish a correspondence between the formula and the name of the corresponding oxide:
Formula
Oxide name
A) CuO
B) Na2O
C) Mn2O7
D) SO3
1) copper (II) oxide
2) sodium oxide
3) sulfur oxide (VI)
4) manganese (II) oxide
5) potassium oxide
6) manganese oxide (VII)
Establish a correspondence between the oxide formula and the reagents with each of which it can interact.
Formula
Reagents
A) Na2O
B) CO2
C) Al2O3
1) HCl, Na(OH), K2CO3
2) H2O, CaO, Ca(OH)2
3) CO2, H2SO4, H2O
4) CaCl2, SiO2, K2O
Vanadium(V) oxide is obtained by burning metal powder in oxygen. Calculate the mass of vanadium required to obtain an oxide mass of 50 g.
Another task number 3 in the test, which I do not understand. Chemical formulas are given: a) MnO c) Fe e) 3SO3 g)Small letters, I presented in the form of numbers: z - 3; z - 2.
Complete task 1,2,3... by selecting one or more correct answers under the letters a, b, c...
1. What letter denotes two chlorine molecules?
2. Which entry reads magnesium-chlorine-two?
3. Choose simple substances.
4. Select formulas of substances with index "1" only.
5. Select the formula of complex substances with coefficient "1" only.
6. Find formulas with chemical elements of side subgroups of D.I. Mendeleev.
7. Find formulas with chemical elements of the third period.
8 Determine the complex substance whose relative molecular weight is 170.
And help me figure this out. exercise:
Using the data from task 3, answer the questions.
What formulas contain the same number of atoms of the same chemical element?
Chemical formula is an image with symbols .
Signs of chemical elements
chemical sign or element chemical symbol is the first or two first letters of the Latin name of this element.
For example: Ferrum-Fe , cuprum-Cu , oxygenium-O etc.
Table 1: Information provided by the chemical mark
| Intelligence | On the example of Cl |
| Element name | Chlorine |
| Non-metal, halogen | |
| One item | 1 chlorine atom |
| (ar) given element | Ar(Cl) = 35.5 |
| Absolute atomic mass of a chemical element
m = Ar 1.66 10 -24 g = Ar 1.66 10 -27 kg |
M (Cl) \u003d 35.5 1.66 10 -24 \u003d 58.9 10 -24 g |
The name of a chemical sign in most cases is read as the name of a chemical element. For example, K - potassium, Ca - calcium, Mg - magnesium, Mn - manganese.
Cases where the name of the chemical mark is read differently are given in Table 2:
| Name of the chemical element | chemical sign | The name of the chemical symbol
(pronunciation) |
| Nitrogen | N | En |
| Hydrogen | H | Ash |
| Iron | Fe | Ferrum |
| Gold | Au | Aurum |
| Oxygen | O | ABOUT |
| Silicon | Si | Silicium |
| Copper | Cu | Cuprum |
| Tin | sn | Stanum |
| Mercury | hg | hydrargium |
| Lead | Pb | Plumbum |
| Sulfur | S | Es |
| Silver | Ag | Argentum |
| Carbon | C | Tse |
| Phosphorus | P | Pe |
Chemical formulas of simple substances
The chemical formulas of most simple substances (all metals and many non-metals) are the signs of the corresponding chemical elements.
So substance iron And chemical element iron are labeled the same Fe .
If it has a molecular structure (exists in the form , then its formula is the chemical sign of the element with index bottom right, indicating number of atoms in a molecule: H2, O2, O 3, N 2, F2, Cl2, Br2, P4, S8.
Table 3: Information provided by the chemical mark
| Intelligence | For example C |
| Substance name | Carbon (diamond, graphite, graphene, carbine) |
| Belonging of an element to a given class of chemical elements | Non-metal |
| One element atom | 1 carbon atom |
| Relative atomic mass (ar) the element that makes up the substance | Ar(C)=12 |
| Absolute atomic mass | M (C) \u003d 12 1.66 10-24 \u003d 19.93 10 -24 g |
| One substance | 1 mole of carbon, i.e. 6.02 10 23 carbon atoms |
| M(C) = Ar(C) = 12 g/mol |
Chemical formulas of complex substances
The formula of a complex substance is compiled by writing the signs of the chemical elements of which this substance consists, indicating the number of atoms of each element in the molecule. In this case, as a rule, chemical elements are written in order of increasing electronegativity according to the following practice series:
Me , Si , B , Te , H , P , As , I , Se , C , S , Br , Cl , N , O , F
For example, H2O , CaSO4 , Al2O3 , CS2 , OF 2 , NaH.
The exception is:
- some compounds of nitrogen with hydrogen (for example, ammonia NH3 , hydrazine N 2H4 );
- salts of organic acids (for example, sodium formate HCOONa , calcium acetate (CH 3COO) 2Ca) ;
- hydrocarbons ( CH 4 , C 2 H 4 , C 2 H 2 ).
Chemical formulas of substances that exist in the form dimers (NO 2 , P2O 3 , P2O5, monovalent mercury salts, for example: HgCl , HgNO3 etc.), is written in the form N 2 O 4 ,P4 O 6 ,P4 O 10 ,Hg 2 Cl2,Hg 2 ( NO 3) 2 .
The number of atoms of a chemical element in a molecule and a complex ion is determined based on the concept valency or oxidation states and recorded index bottom right from the sign of each element (index 1 is omitted). This is based on the rule:
the algebraic sum of the oxidation states of all atoms in a molecule must be equal to zero (the molecules are electrically neutral), and in a complex ion, the charge of the ion.
For example:
2Al 3 + + 3SO 4 2- \u003d Al 2 (SO 4) 3
The same rule is used when determining the degree of oxidation of a chemical element according to the formula of a substance or complex. Usually it is an element that has several oxidation states. The oxidation states of the remaining elements forming the molecule or ion must be known.
The charge of a complex ion is the algebraic sum of the oxidation states of all the atoms that form the ion. Therefore, when determining the oxidation state of a chemical element in a complex ion, the ion itself is enclosed in brackets, and its charge is taken out of brackets.
When compiling formulas for valency the substance is represented as a compound consisting of two particles of different types, the valences of which are known. Further enjoy rule:
in a molecule, the product of valence and the number of particles of one type must be equal to the product of valence and the number of particles of another type.
For example:
The number in front of a formula in a reaction equation is called coefficient. She indicates either number of molecules, or number of moles of a substance.
The coefficient before the chemical sign, indicates the number of atoms of a given chemical element, and in the case when the sign is a formula of a simple substance, the coefficient indicates either number of atoms, or the number of moles of this substance.
For example:
- 3 Fe- three iron atoms, 3 moles of iron atoms,
- 2 H- two hydrogen atoms, 2 mol hydrogen atoms,
- H2- one molecule of hydrogen, 1 mole of hydrogen.
The chemical formulas of many substances have been determined empirically, which is why they are called "empirical".
Table 4: Information provided by the chemical formula of a complex substance
| Intelligence | For example C aCO3 |
| Substance name | Calcium carbonate |
| Belonging of an element to a certain class of substances | Medium (normal) salt |
| One molecule of a substance | 1 molecule of calcium carbonate |
| One mole of a substance | 6.02 10 23 molecules CaCO3 |
| Relative molecular weight of the substance (Mr) | Mr (CaCO3) \u003d Ar (Ca) + Ar (C) + 3Ar (O) \u003d 100 |
| Molar mass of a substance (M) | M (CaCO3) = 100 g/mol |
| Absolute molecular weight of a substance (m) | M (CaCO3) = Mr (CaCO3) 1.66 10 -24 g = 1.66 10 -22 g |
| Qualitative composition (what chemical elements form a substance) | calcium, carbon, oxygen |
| The quantitative composition of the substance: | |
| The number of atoms of each element in one molecule of a substance: | The calcium carbonate molecule is made up of 1 atom calcium, 1 atom carbon and 3 atoms oxygen. |
| The number of moles of each element in 1 mole of a substance: | In 1 mol CaCO 3(6.02 10 23 molecules) contains 1 mol(6.02 10 23 atoms) calcium, 1 mol(6.02 10 23 atoms) carbon and 3 mol(3 6.02 10 23 atoms) of the chemical element oxygen) |
| Mass composition of the substance: | |
| The mass of each element in 1 mole of a substance: | 1 mole of calcium carbonate (100g) contains chemical elements: 40g calcium, 12g carbon, 48g oxygen. |
| Mass fractions of chemical elements in a substance (composition of a substance in percent by weight):
|
Composition of calcium carbonate by mass:
W (Ca) \u003d (n (Ca) Ar (Ca)) / Mr (CaCO3) \u003d (1 40) / 100 \u003d 0.4 (40%) W (C) \u003d (n (Ca) Ar (Ca)) / Mr (CaCO3) \u003d (1 12) / 100 \u003d 0.12 (12%) W (O) \u003d (n (Ca) Ar (Ca)) / Mr (CaCO3) \u003d (3 16) / 100 \u003d 0.48 (48%) |
| For a substance with an ionic structure (salts, acids, bases) - the formula of a substance gives information about the number of ions of each type in a molecule, their number and mass of ions in 1 mol of a substance:
|
Molecule CaCO 3 is made up of an ion Ca 2+ and ion CO 3 2-
1 mol ( 6.02 10 23 molecules) CaCO 3 contains 1 mol of Ca 2+ ions And 1 mole of ions CO 3 2-; 1 mole (100g) of calcium carbonate contains 40g ions Ca 2+ And 60g ions CO 3 2- |
| Molar volume of a substance under normal conditions (only for gases) | |
Graphic formulas
For more complete information about the substance used graphic formulas , which indicate the order in which atoms are connected in a molecule And valency of each element.
Graphic formulas of substances consisting of molecules, sometimes, to one degree or another, reflect the structure (structure) of these molecules, in these cases they can be called structural .
To draw up a graphical (structural) formula of a substance, you must:
- Determine the valence of all chemical elements that form a substance.
- Write down the signs of all chemical elements that form a substance, each in an amount equal to the number of atoms of a given element in a molecule.
- Connect the signs of chemical elements with dashes. Each line denotes a pair that makes a connection between chemical elements and therefore equally belongs to both elements.
- The number of dashes surrounding the sign of a chemical element must correspond to the valence of this chemical element.
- When formulating oxygen-containing acids and their salts, hydrogen atoms and metal atoms are bound to the acid-forming element through an oxygen atom.
- Oxygen atoms are connected to each other only when formulating peroxides.
Examples of graphic formulas:
