What information can be obtained from a chemical formula. Dictionary of chemical formulas

What is a chemical formula? Every science has its own notation. Chemistry is no exception in this regard. You already know that symbols derived from the Latin names of 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, it is necessary to write down the symbol of the chemical element that forms the simple substance, and write down a number at the bottom right showing the number of its atoms. This number is called index. For example, the chemical formula for oxygen is O2. The number 2 after the symbol for oxygen 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 what atoms of which elements it consists of (qualitative composition), and the number of atoms of each element (quantitative composition). For example, the chemical formula for 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, carbon atoms, and three oxygen atoms. This is the quantitative composition of soda Qualitative composition substance shows which atoms of which elements are included in its composition Quantitative composition substance shows the number of atoms that make up its composition Chemical formula- conditional recording of the composition of a substance using chemical symbols and indices Please note that if there is only one atom of the same type in the chemical formula, the index 1 is not put. For example, the formula for carbon dioxide is written like this: CO2, not C1O2.

How to understand chemical formulas correctly?

The law of constancy of the composition of matter

• Index- a number showing the number of atoms of a certain type in a chemical formula
• The qualitative composition of a substance shows the atoms of which elements are included in its composition.
• The quantitative composition of a substance shows the number of atoms that make up its composition.
• Chemical formula - a conditional record of the composition of a substance using chemical symbols and indices (if necessary)
• Coefficient is 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 an atomic or ionic structure

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

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:

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

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 valency 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

Graphic formulas

For more information about a substance use 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:

Chemical formulas are used to designate substances.

Chemical formula- this is a conditional record of the composition of a substance by means of chemical signs and indices.

With the help 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 in front of the formula of a substance is called the coefficient and indicates the number of molecules of this substance. For example, 4H 2 O - 4 water molecules. Four water molecules contain 8 hydrogen atoms and 4 oxygen atoms.

Information given by the chemical formula of a substance

Using the example of carbon dioxide CO2, 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 on the basis of data obtained experimentally. If the mass fractions of elements in a substance and the relative molecular mass of a substance are known, the number of atoms of each element in a 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. Write the chemical formula for carbon dioxide. For this you need:

1. determine the mass per share of oxygen atoms in a 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. make a formula for carbon dioxide: CO 2.

Literature

1. Collection of tasks and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. "Chemistry, Grade 8" / 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. Grade 8” / 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 institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§14)

4. Chemistry: inorg. chemistry: textbook. for 8 cells. general institutions / G.E. Rudzitis, FuGyu Feldman. - M .: Education, JSC "Moscow Textbooks", 2009. (§10)

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

The lesson is devoted to the study of the rules for compiling and reading chemical formulas of substances. You will learn what information the chemical formula of a substance gives and how to draw up a chemical formula based on data on the mass fractions of chemical elements.

Topic: Initial chemical ideas

Lesson: The chemical formula of a substance

1. Chemical formula of the substance

Chemical formulas are used to designate substances.

Chemical formula is a conditional record of the composition of a substance by means of chemical signs and indices.

With the help of 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 consists of one carbon atom and two oxygen atoms - CO2. An index equal to one is not written.

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

2. Information given by the chemical formula of a substance

Using the example of carbon dioxide CO2, 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

Chemical formulas are derived on the basis of data obtained experimentally. If the mass fractions of elements in a substance and the relative molecular mass of a substance are known, the number of atoms of each element in a 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. Write the chemical formula for carbon dioxide. For this you need:

1. determine the mass per share of oxygen atoms in a 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. make a formula for carbon dioxide: CO2.

1. Collection of tasks and exercises in chemistry: 8th grade: to the textbook by P. A. Orzhekovsky and others “Chemistry, grade 8” / P. A. Orzhekovsky, N. A. Titov, F. F. Hegele. - 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. Grade 8» / 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 institutions / P. A. Orzhekovsky, L. M. Meshcheryakova, L. S. Pontak. M.: AST: Astrel, 2005.(§14)

4. Chemistry: inorg. chemistry: textbook. for 8 cells. general institutions / G. E. Rudzitis, FuGyu Feldman. - M.: Enlightenment, JSC "Moscow textbooks", 2009. (§10)

5. Encyclopedia for children. Volume 17. Chemistry / Chapter. ed. V.A. Volodin, leading. 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".

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 №№ 3,4,6,7 from the 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.

>> Chemical formulas

Chemical formulas

The paragraph will help you:

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

Chemical formula.

Everyone has it substances there is a name. However, by name it is impossible to determine what particles the substance consists of, how many and what atoms are contained in its molecules, ions, what charges the ions have. 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, an aluminum atom is denoted by the symbol Al, and a silicon atom by the symbol Si. Simple substances also have such formulas - the metal aluminum, the non-metal of the atomic structure silicon.

Chemical formula molecule of a simple substance contains the symbol of the corresponding element and a 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 pronouncing first the symbol of the element, then the index: “o-two”. The formula O 2 denotes not only the molecule, but also the substance oxygen itself.

The O 2 molecule is called diatomic. Of these molecules (their general formula is E 2), the simple substances of Hydrogen, Nitrogen, Fluor, Chlorine, Bromine, Iodine are composed.

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

H 2
O2
N 2
Cl2
Br2
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 the indices, are written. The 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 there is one atom of any element in the molecule, then the corresponding index, i.e. I, is not written in the chemical formula. The formula of the carbon dioxide molecule is also the formula of the substance itself.

In the formula of an ion, its charge is additionally recorded. To do this, use the superscript. In it, a number indicates the amount of charge (they do not write a unit), and then a sign (plus or minus). For example, a Sodium ion with a charge of +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 of -2 - CO 2- 3 ("tse-o-three-two-minus").

Na + , Cl -
simple ions

OH -, CO 2- 3
complex ions

In the formulas of ionic compounds, they 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 equal to zero.

table 2
Formulas of some ionic compounds

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

Sometimes in chemical formulas, instead of the symbols of the elements, "foreign" letters are written, as well as index letters. Such formulas are often called general. Examples of formulas of this type: ECI n , E n O m , Fe 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 the Ferrum compound 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 notation 2Ne, 20 2, 2C0 2, 2Na +. The number in front of the chemical formula is called the coefficient. The coefficient I, like the index I, is not written.

formula unit.

What does 2NaCl mean? There are no NaCl molecules; table salt is an ionic compound that consists of Na + and Cl - ions. A pair of these ions is called the formula unit of matter (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 silicon 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 "brick" of a substance, its smallest repeating fragment. This fragment can be an atom (in simple matter), molecule(in simple or complex matter),
a collection of atoms or ions (in a complex substance).

An exercise. Compose the chemical formula of 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 there are two Li + ions for each SO 2- 4 ion. 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 the substance.

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

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

Formula unit - a particle or a 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 records? Complete your answer with examples. What is 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, ZCa (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-twice-es; barium-two-plus; pe-o-four-three-minus.

71. Make a chemical formula of a molecule that contains: a) one Nitrogen atom and three Hydrogen atoms; b) four hydrogen atoms, two phosphorus atoms and seven oxygen atoms.

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 formulas for all substances that can contain only such ions: K + , Mg2 + , F - , SO -2 4 , OH - .

74. Describe the qualitative and quantitative composition:

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) H 3 O +, HPO 2- 4 ions.

Popel P. P., Kriklya L. S., Chemistry: Pdruch. for 7 cells. zahalnosvit. navch. zakl. - K .: Exhibition Center "Academy", 2008. - 136 p.: il.