The chemical formula of a substance gives the following information. Chemical formulas of substances. Chemical formulas of simple substances

Chemical formulas are used to designate substances.

Chemical formula is a conventional notation of the composition of a substance using chemical symbols and indices.

Using the indices of Y.Ya. Berzelius proposed to designate the number of atoms of a chemical element in a molecule of a substance. For example: a water molecule contains two hydrogen atoms and one oxygen atom - H 2 O (2 is the index). Carbon dioxide contains one carbon atom and two oxygen atoms – CO 2 . An index equal to one is not written.

The number before the formula of a substance is called a coefficient and indicates the number of molecules of a given substance. For example, 4H 2 O – 4 water molecules. Four water molecules contain 8 hydrogen atoms and 4 oxygen atoms.

Information provided by the chemical formula of a substance

Using carbon dioxide CO2 as an example, let’s consider what information about a substance can be obtained from its chemical formula.

Based on the chemical formula, you can calculate the mass fractions of chemical elements in a substance; this will be discussed in the material of the next lesson.

Derivation of the chemical formula of a substance

Chemical formulas are derived based on data obtained experimentally. If the mass fractions of elements in a substance and the relative molecular weight of the substance are known, the number of atoms of each element in the molecule can be found.

Example. It is known that the relative molecular weight of carbon dioxide is 44. The mass fraction of oxygen in this substance is 0.727 (72.7%), the rest is carbon. Let's make up the chemical formula of carbon dioxide. To do this you need:

1. determine the mass per share of oxygen atoms in the molecule:

44*0.727=32 (relative units);

2. determine the number of oxygen atoms, knowing that the relative atomic mass of oxygen is 16:

3. determine the mass per share of carbon atoms:

44-32=12 (relative units);

4. determine the number of carbon atoms, knowing that the relative atomic mass of carbon is 12:

5. create the formula for carbon dioxide: CO 2.

Literature

1. Collection of problems and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry, 8th grade” / P.A. Orzhekovsky, N.A. Titov, F.F. Hegel. – M.: AST: Astrel, 2006. (p.26-28)

2. Ushakova O.V. Chemistry workbook: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. 8th grade” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006. (p. 32-34)

3. Chemistry: 8th grade: textbook. for general education institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§14)

4. Chemistry: inorg. chemistry: textbook. for 8th grade. general education institutions / G.E. Rudzitis, Fyu Feldman. – M.: Education, OJSC “Moscow Textbooks”, 2009. (§10)

5. Encyclopedia for children. Volume 17. Chemistry / Chapter. ed.V.A. Volodin, Ved. scientific ed. I. Leenson. – M.: Avanta, 2003.

The lesson is devoted to learning the rules of drawing up and reading chemical formulas of substances. You will learn what information the chemical formula of a substance provides and how to compose a chemical formula based on data on the mass fractions of chemical elements.

Topic: Initial chemical ideas

Lesson: Chemical formula of a substance

1. Chemical formula of the substance

Chemical formulas are used to designate substances.

Chemical formula is a conventional notation of the composition of a substance using chemical signs And indexes.

Using indices, J. Ya. Berzelius proposed to designate the number of atoms of a chemical element in a molecule of a substance. For example: a water molecule contains two hydrogen atoms and one oxygen atom - H2O (2 - index). Carbon dioxide contains one carbon atom and two oxygen atoms - CO2. An index equal to one is not written.

The number before the formula of a substance is called a coefficient and indicates the number of molecules of a given substance. For example, 4H2O - 4 water molecules. Four water molecules contain 8 hydrogen atoms and 4 oxygen atoms.

2. Information provided by the chemical formula of a substance

Using carbon dioxide CO2 as an example, let’s consider what information about a substance can be obtained from its chemical formula.

Table 1.

Based on the chemical formula, you can calculate the mass fractions of chemical elements in a substance; this will be discussed in the material of the next lesson.

3. Derivation of the chemical formula of a substance

Example. It is known that the relative molecular weight of carbon dioxide is 44. The mass fraction of oxygen in this substance is 0.727 (72.7%), the rest is carbon. Let's make up the chemical formula of carbon dioxide. To do this you need:

1. determine the mass per share of oxygen atoms in the molecule:

440.727=32 (relative units);

2. determine the number of oxygen atoms, knowing that the relative atomic mass of oxygen is 16:

3. determine the mass per share of carbon atoms:

44-32=12 (relative units);

4. determine the number of carbon atoms, knowing that the relative atomic mass of carbon is 12:

5. create the formula for carbon dioxide: CO2.

1. Collection of problems and exercises in chemistry: 8th grade: to the textbook by P. A. Orzhekovsky and others “Chemistry, 8th grade” / P. A. Orzhekovsky, N. A. Titov, F. F. Hegele. - M.: AST: Astrel, 2006. (p.26-28)

2. Ushakova O. V. Workbook on chemistry: 8th grade: to the textbook by P. A. Orzhekovsky and others “Chemistry. 8th grade” / O. V. Ushakova, P. I. Bespalov, P. A. Orzhekovsky; under. ed. prof. P. A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006. (p. 32-34)

3. Chemistry: 8th grade: textbook. for general education institutions / P. A. Orzhekovsky, L. M. Meshcheryakova, L. S. Pontak. M.: AST: Astrel, 2005.(§14)

4. Chemistry: inorg. chemistry: textbook. for 8th grade. general education institutions / G. E. Rudzitis, Fyu Feldman. - M.: Education, OJSC “Moscow Textbooks”, 2009. (§10)

5. Encyclopedia for children. Volume 17. Chemistry / Chapter. ed. V.A. Volodin, Ved. scientific ed. I. Leenson. - M.: Avanta+, 2003.

Additional web resources

1. A unified collection of digital educational resources.

2. Electronic version of the journal “Chemistry and Life”.

3. Chemistry tests (online).

Homework

1. p.77 No. 3 from the textbook “Chemistry: 8th grade” (P. A. Orzhekovsky, L. M. Meshcheryakova, L. S. Pontak. M.: AST: Astrel, 2005).

2. With. 32-34 No. 3,4,6,7 from the Workbook on Chemistry: 8th grade: to the textbook by P. A. Orzhekovsky and others “Chemistry. 8th grade” / O. V. Ushakova, P. I. Bespalov, P. A. Orzhekovsky; under. ed. prof. P. A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006.

Topic: Initial chemical ideas

Lesson: Chemical formula of a substance

Chemical formulas are used to designate substances.

Chemical formula is a conventional notation of the composition of a substance using chemical signs And indexes.

Using the indices of Y.Ya. Berzelius proposed to designate the number of atoms of a chemical element in a molecule of a substance. For example: a water molecule contains two hydrogen atoms and one oxygen atom - H 2 O (2 - index). Carbon dioxide contains one carbon atom and two oxygen atoms - CO 2. An index equal to one is not written.

The number before the formula of a substance is called coefficient and indicates the number of molecules of a given substance. For example, 4H 2 O - 4 water molecules. Four water molecules contain 8 hydrogen atoms and 4 oxygen atoms.

Using carbon dioxide CO 2 as an example, let’s consider what information about a substance can be obtained from its chemical formula.

Table 1.

Based on the chemical formula, you can calculate the mass fractions of chemical elements in a substance; this will be discussed in the material of the next lesson.

Chemical formulas are derived based on data obtained experimentally. If the elements in a substance and the relative substance are known, one can find the number of atoms of each element in the molecule.

1. determine the mass per share of oxygen atoms in the molecule:

44*0.727=32 (relative units);

2. determine the number of oxygen atoms, knowing that the relative atomic mass of oxygen is 16:

3. determine the mass per share of carbon atoms:

44-32=12 (relative units);

4. determine the number of carbon atoms, knowing that the relative atomic mass of carbon is 12:

5. create the formula for carbon dioxide: CO 2.

3. Chemistry: 8th grade: textbook. for general education institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§14)

4. Chemistry: inorg. chemistry: textbook. for 8th grade. general education institutions / G.E. Rudzitis, Fyu Feldman. - M.: Education, OJSC “Moscow Textbooks”, 2009. (§10)

5. Encyclopedia for children. Volume 17. Chemistry / Chapter. ed.V.A. Volodin, Ved. scientific ed. I. Leenson. - M.: Avanta+, 2003.

Additional web resources

1. Unified collection of digital educational resources ().

2. Electronic version of the journal “Chemistry and Life” ().

Homework

1. p.77 No. 3 from the textbook “Chemistry: 8th grade” (P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005).

2. With. 32-34 No. 3,4,6,7 from the Workbook in Chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. 8th grade” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006.

What is the chemical formula? Every science has its own notation system. Chemistry is no exception in this regard. You already know that symbols derived from the Latin names of the elements are used to designate chemical elements. Chemical elements are capable of forming both simple and complex substances, the composition of which can be expressed chemical formula. To write the chemical formula of a simple substance, you need to write down the symbol of the chemical element that forms the simple substance, and write down the number at the bottom right showing the number of its atoms. This figure is called index. For example, the chemical formula of oxygen is O2. The number 2 after the oxygen symbol is an index indicating that the oxygen molecule consists of two atoms of the element oxygen. Index - a number showing the number of atoms of a certain type in a chemical formula To write the chemical formula of a complex substance, you need to know which atoms of elements it consists of (qualitative composition), and the number of atoms of each element (quantitative composition). For example, the chemical formula of baking soda is NaHCO3. The composition of this substance includes atoms of sodium, hydrogen, carbon, oxygen - this is its qualitative composition. There are one sodium, hydrogen, and carbon atoms each, and three oxygen atoms. This is the quantitative composition of soda High-quality composition a substance shows which atoms of elements are included in its composition Quantitative composition a substance shows the number of atoms that make up it Chemical formula– conventional recording of the composition of a substance using chemical symbols and indices Please note that if a chemical formula contains only one atom of one type, the subscript 1 is not used. For example, the formula for carbon dioxide is written as follows: CO2, not C1O2.

How to understand chemical formulas correctly?

When writing chemical formulas, you often come across numbers that are written before the chemical formula. For example, 2Na, or 5O2. What do these numbers mean and what are they for? The numbers written before a chemical formula are called coefficients. Coefficients show the total number of particles of a substance: atoms, molecules, ions. For example, the notation 2Na represents two sodium atoms. The notation 5O2 means five/ Coefficient - a number that shows the total number of particles. The coefficient is written before the chemical formula of the substance of oxygen molecules. Please note that molecules cannot consist of one atom, the minimum number of atoms in a molecule is two. Thus the entries: 2H, 4P stand for two hydrogen atoms and four phosphorus atoms, respectively. Record 2H2 denotes two hydrogen molecules containing two atoms of the element hydrogen. Record 4S8– denotes four sulfur molecules, each containing eight atoms of the element sulfur. A similar system of notation for the number of particles is used for ions. Record 5K+ stands for five potassium ions. It is worth noting that ions can be formed not only by an atom of one element. Ions formed by atoms of one chemical element are called simple: Li+, N3−. Ions formed by several chemical elements are called complex: OH⎺, SO4 2−. Note that the charge of an ion is indicated by a superscript. What will the entry mean? 2NaCl? If the answer to this question is two molecules of table salt, then the answer is not correct. Table salt, or sodium chloride, has an ionic crystal lattice, that is, it is an ionic compound and consists of ions Na+ and Cl⎺. A pair of these ions is called formula unit of a substance. Thus, the notation 2NaCl means two formula units sodium chloride. The term formula unit is also used for substances of atomic structure. Formula unit– the smallest particle of a substance of non-molecular structure Ionic compounds are just as electrically neutral as molecular ones. This means that the positive charge of the cations is completely balanced by the negative charge of the anions. For example, what is the formula unit of a substance consisting of ions Ag+ and PO4 3−? Obviously, to compensate for the negative charge of the ion (charge –3), it is necessary to have a charge of +3. Taking into account the fact that the silver cation has a charge of +1, then three such cations are needed. This means the formula unit (formula) of a given substance is Ag3PO4. Thus, using the symbols of chemical elements, indices and coefficients, it is possible to clearly compose the chemical formula of a substance, which will provide information about both the qualitative and quantitative composition of the substance. Finally, let's look at how to correctly pronounce chemical formulas. For example, record 3Ca2+ pronounced: "three calcium ions two plus" or “three calcium ions with charge two plus.” Record 4HCl, pronounced “four molecules of ash chlorine.” Record 2NaCl, pronounced like "two formula units of sodium chloride."

Law of constancy of the composition of matter

The same chemical compound can be prepared in different ways. For example, carbon dioxide CO2, is formed by burning fuels: coal, natural gas. Fruits contain a lot of glucose. During long-term storage, fruits begin to deteriorate and a process called glucose fermentation begins, which results in the release of carbon dioxide. Carbon dioxide is also formed when rocks such as chalk, marble, and limestone are heated. Chemical reactions are completely different, but the substance formed as a result of their occurrence has the same qualitative and quantitative composition – CO2. This pattern applies mainly to substances of molecular structure. In the case of substances of non-molecular structure, there may be cases when the composition of the substance depends on the methods of its preparation. The law of constancy of the composition of substances of molecular structure: the composition of a complex substance is always the same and does not depend on the method of its preparation Conclusions from an article on the topic Chemical formulas of substances

Index– a number showing the number of atoms of a certain type in a chemical formula
The qualitative composition of a substance shows which atoms of elements are included in its composition
The quantitative composition of a substance shows the number of atoms that are included in its composition
Chemical formula - a conventional recording of the composition of a substance using chemical symbols and indices (if necessary)
Coefficient– a number that shows the total number of particles. The coefficient is written before the chemical formula of the substance
Formula unit– the smallest particle of a substance of atomic or ionic structure

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>> Chemical formulas

Chemical formulas

The material in this paragraph will help you:

> find out what the chemical formula is;
> read the formulas of substances, atoms, molecules, ions;
> use the term “formula unit” correctly;
> compose chemical formulas of ionic compounds;
> characterize the composition of a substance, molecule, ion using a chemical formula.

Chemical formula.

Everyone has it substances there is a name. However, by its name it is impossible to determine what particles a substance consists of, how many and what kind of atoms are contained in its molecules, ions, and what charges the ions have. The answers to such questions are given by a special record - a chemical formula.

A chemical formula is the designation of an atom, molecule, ion or substance using symbols chemical elements and indexes.

The chemical formula of an atom is the symbol of the corresponding element. For example, the Aluminum atom is designated by the symbol Al, the Silicon atom by the symbol Si. Simple substances also have such formulas - the metal aluminum, the non-metal of atomic structure silicon.

Chemical formula molecules of a simple substance contains the symbol of the corresponding element and the subscript - a small number written below and to the right. The index indicates the number of atoms in the molecule.

An oxygen molecule consists of two oxygen atoms. Its chemical formula is O 2. This formula is read by first pronouncing the symbol of the element, then the index: “o-two”. The formula O2 denotes not only the molecule, but also the substance oxygen itself.

The O2 molecule is called diatomic. The simple substances Hydrogen, Nitrogen, Fluor, Chlorine, Bromine, and Iodine consist of similar molecules (their general formula is E 2).

Ozone contains three-atomic molecules, white phosphorus contains four-atomic molecules, and sulfur contains eight-atomic molecules. (Write the chemical formulas of these molecules.)

H 2
O2
N 2
Cl2
BR 2
I 2

In the formula of a molecule of a complex substance, the symbols of the elements whose atoms are contained in it, as well as indices, are written down. A carbon dioxide molecule consists of three atoms: one carbon atom and two oxygen atoms. Its chemical formula is CO 2 (read “tse-o-two”). Remember: if a molecule contains one atom of any element, then the corresponding index, i.e. I, is not written in the chemical formula. The formula of a carbon dioxide molecule is also the formula of the substance itself.

In the formula of an ion, its charge is additionally written down. To do this, use a superscript. It indicates the amount of charge with a number (they do not write one), and then a sign (plus or minus). For example, a Sodium ion with a charge +1 has the formula Na + (read “sodium-plus”), a Chlorine ion with a charge - I - SG - (“chlorine-minus”), a hydroxide ion with a charge - I - OH - (“ o-ash-minus"), a carbonate ion with a charge -2 - CO 2- 3 (“ce-o-three-two-minus”).

Na+,Cl-
simple ions

OH - , CO 2- 3
complex ions

In the formulas of ionic compounds, first write down, without indicating charges, positively charged ions, and then - negatively charged (Table 2). If the formula is correct, then the sum of the charges of all ions in it is zero.

table 2
Formulas of some ionic compounds

In some chemical formulas, a group of atoms or a complex ion is written in parentheses. As an example, let's take the formula of slaked lime Ca(OH) 2. This is an ionic compound. In it, for every Ca 2+ ion there are two OH - ions. The formula of the compound reads " calcium-o-ash-twice”, but not “calcium-o-ash-two”.

Sometimes in chemical formulas, instead of symbols of elements, “foreign” letters, as well as index letters, are written. Such formulas are often called general. Examples of formulas of this type: ECI n, E n O m, F x O y. First
the formula denotes a group of compounds of elements with Chlorine, the second - a group of compounds of elements with Oxygen, and the third is used if the chemical formula of a compound of Ferrum with Oxygen unknown and
it should be installed.

If you need to designate two separate Neon atoms, two oxygen molecules, two carbon dioxide molecules or two Sodium ions, use the notations 2Ne, 20 2, 2C0 2, 2Na +. The number in front of the chemical formula is called the coefficient. Coefficient I, like index I, is not written.

Formula unit.

What does the notation 2NaCl mean? NaCl molecules do not exist; table salt is an ionic compound that consists of Na + and Cl - ions. A pair of these ions is called the formula unit of a substance (it is highlighted in Fig. 44, a). Thus, the notation 2NaCl represents two formula units of table salt, i.e., two pairs of Na + and C l- ions.

The term “formula unit” is used for complex substances not only of ionic but also of atomic structure. For example, the formula unit for quartz SiO 2 is the combination of one Silicium atom and two Oxygen atoms (Fig. 44, b).

Rice. 44. formula units in compounds of ionic (a) atomic structure (b)

A formula unit is the smallest “building block” of a substance, its smallest repeating fragment. This fragment can be an atom (in a simple substance), molecule(in a simple or complex substance),
a collection of atoms or ions (in a complex substance).

Exercise. Draw up a chemical formula for a compound that contains Li + i SO 2- 4 ions. Name the formula unit of this substance.

Solution

In an ionic compound, the sum of the charges of all ions is zero. This is possible provided that for each SO 2- 4 ion there are two Li + ions. Hence the formula of the compound is Li 2 SO 4.

The formula unit of a substance is three ions: two Li + ions and one SO 2- 4 ion.

Qualitative and quantitative composition of a substance.

A chemical formula contains information about the composition of a particle or substance. When characterizing the qualitative composition, they name the elements that form a particle or substance, and when characterizing the quantitative composition, they indicate:

The number of atoms of each element in a molecule or complex ion;
the ratio of atoms of different elements or ions in a substance.

Exercise. Describe the composition of methane CH 4 (molecular compound) and soda ash Na 2 CO 3 (ionic compound)

Solution

Methane is formed by the elements Carbon and Hydrogen (this is a qualitative composition). A methane molecule contains one carbon atom and four hydrogen atoms; their ratio in the molecule and in the substance

N(C): N(H) = 1:4 (quantitative composition).

(The letter N denotes the number of particles - atoms, molecules, ions.

Soda ash is formed by three elements - Sodium, Carbon and Oxygen. It contains positively charged Na + ions, since Sodium is a metallic element, and negatively charged CO -2 3 ions (qualitative composition).

The ratio of atoms of elements and ions in a substance is as follows:

conclusions

A chemical formula is a recording of an atom, molecule, ion, substance using symbols of chemical elements and indices. The number of atoms of each element is indicated in the formula using a subscript, and the charge of the ion is indicated by a superscript.

Formula unit is a particle or collection of particles of a substance represented by its chemical formula.

The chemical formula reflects the qualitative and quantitative composition of a particle or substance.

?
66. What information about a substance or particle does a chemical formula contain?

67. What is the difference between a coefficient and a subscript in chemical notation? Complete your answer with examples. What is the superscript used for?

68. Read the formulas: P 4, KHCO 3, AI 2 (SO 4) 3, Fe(OH) 2 NO 3, Ag +, NH + 4, CIO - 4.

69. What do the entries mean: 3H 2 0, 2H, 2H 2, N 2, Li, 4Cu, Zn 2+, 50 2-, NO - 3, 3Ca(0H) 2, 2CaC0 3?

70. Write down chemical formulas that read like this: es-o-three; boron-two-o-three; ash-en-o-two; chrome-o-ash-thrice; sodium-ash-es-o-four; en-ash-four-double-es; barium-two-plus; pe-o-four-three-minus.

71. Make up the chemical formula of a molecule that contains: a) one Nitrogen atom and three Hydrogen atoms; b) four atoms of Hydrogen, two atoms of Phosphorus and seven atoms of Oxygen.

72. What is the formula unit: a) for soda ash Na 2 CO 3 ; b) for the ionic compound Li 3 N; c) for the compound B 2 O 3, which has an atomic structure?

73. Make up formulas for all substances that can only contain the following ions: K + , Mg2 + , F - , SO -2 4 , OH - .

74. Describe the qualitative and quantitative composition of:

a) molecular substances - chlorine Cl 2, hydrogen peroxide (hydrogen peroxide) H 2 O 2, glucose C 6 H 12 O 6;
b) ionic substance - sodium sulfate Na 2 SO 4;
c) ions H 3 O +, HPO 2- 4.

Popel P. P., Kryklya L. S., Chemistry: Pidruch. for 7th grade zagalnosvit. navch. closing - K.: VC "Academy", 2008. - 136 p.: ill.

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