One of the most important tasks in chemistry is the correct composition of chemical formulas. A chemical formula is a written representation of a composition chemical using the Latin element designation and indices. To draw up a formula correctly, we will definitely need a periodic table and knowledge of simple rules. They are quite simple and even children can remember them.
How to write chemical formulas
The main concept in the preparation of chemical formulas is "valence". Valence is the property of one element to hold a certain number of atoms in a compound. The valence of a chemical element can be viewed in the periodic table, and you also need to remember and be able to apply simple general rules.
- The valency of the metal is always equal to the group number, provided that it is in the main subgroup. For example, potassium has a valency of 1 and calcium has a valence of 2.
- Non-metals are a little more complicated. Non-metal can have higher and lower valence. The highest valency is equal to the group number. The lowest valence can be determined by subtracting the element group number from eight. When combined with metals, non-metals always have the lowest valence. Oxygen always has a valency of 2.
- In the combination of two non-metals, the lowest valency has that chemical element, which is located in the periodic table to the right and above. However, fluorine always has a valency of 1.
- And one more important rule when placing odds! The total number of valencies of one element must always be equal to the total number of valencies of another element!
Let's consolidate the knowledge gained by the example of the combination of lithium and nitrogen. Lithium metal has a valency of 1. Non-metal nitrogen is located in group 5 and has a higher valency of 5 and a lower valence of 3. As we already know, in compounds with metals, non-metals always have a lower valence, therefore nitrogen in this case will have a valency of three. We place the coefficients and get the required formula: Li 3 N.
So, quite simply, we learned how to make chemical formulas! And for better memorization of the algorithm for drawing up formulas, we have prepared its graphical representation.
One chemical element to attach or replace a certain number of atoms of another.
The valence unit of the hydrogen atom is taken to be 1, that is, hydrogen is monovalent. Therefore, the valence of an element indicates how many hydrogen atoms one atom of the element in question is connected to. For instance, HCl, where chlorine is monovalent; H2O, where oxygen is bivalent; NH 3 where nitrogen is trivalent.
Table of elements with constant valence.
Formulas of substances can be composed according to the valencies of the elements included in them. And vice versa, knowing the valences of the elements, you can make up a chemical formula from them.
Algorithm for drawing up the formulas of substances by valency.
1. Write down the symbols of the elements.
2. Determine the valencies of the elements included in the formula.
3. Find the smallest common multiple of the numerical values of valence.
4. Find the ratios between the atoms of the elements by dividing the found least common multiple by the corresponding valencies of the elements.
5. Write down the indices of the elements in the chemical formula.
Example: Let's make the chemical formula of phosphorus oxide.
1. Let's write down the symbols:
2. Let's define the valencies:
4. Let's find the relationship between the atoms:
5. Let's write down the indices:
Algorithm for determining the valence by the formulas of chemical elements.
1. Write down the formula of the chemical compound.
2. Designate the known valency of the elements.
3. Find the least common multiple of valence and index.
4. Find the ratio of the least common multiple to the number of atoms of the second element. This is the desired valence.
5. Make a check by multiplying the valence and index of each element. Their works must be equal.
Example: determine the valence of the elements of hydrogen sulfide.
1. Let's write down the formula:
H 2 S
2. Let's denote the known valence:
H 2 S
3. Find the least common multiple:
H 2 S
4. Find the ratio of the least common multiple to the number of sulfur atoms:
H 2 S
5. Let's check.
Valence.
Compilation of chemical formulas
by valence
8th grade
Lesson type. Combined.
Teaching methods. Partially search, reproductive, programmed survey, conversation with lecture elements.
Epigraph to the lesson.“Any substance - from the simplest to the most complex - has three different, but interrelated aspects: properties, composition, structure ...” (BM Kedrov).
Goals. Didactic: consider the concept of "valence" as the atomicity of an element, acquaint students with different kinds valence (highest and lowest, variable and constant).
Psychological: arouse interest in the subject, develop the ability to reason logically, competently express their thoughts.
Educational: develop the ability to work collectively, evaluate the answers of their comrades.
Equipment. Models water molecules, carbon dioxide, sets for building models of molecules of various substances, individual cards for checking homework and independent work of students in a group, anagram plates for a chemical warm-up, a scale for determining emotional state student.
DURING THE CLASSES
Indicative and motivational stage
Psychological warm-up
The purpose of the warm-up is to determine the emotional state of the students. Each student has a plate with six faces glued on the inside of the cover of the notebook - a scale for determining the emotional state (Fig.). Each student places a check mark under the face whose expression reflects his mood.
Teacher. It would be great if by the end of the lesson everyone could move the checkbox at least one cell to the left.
To do this, you need to think about the questions: can a person fall in love with a school subject that is not very interesting to him? What do I need to do?
Chemical warm-up
The warm-up is prepared and carried out by the students.
Student. Anagrams are words in which the order of the letters is changed. Try some of the chemical anagrams. Rearrange the letters in each word and get the name of the chemical element. Pay attention to the hint.
"Odovrod" - this element has the smallest relative atomic mass.
"Mailinui" - this element is called "winged" metal.
"Turt" - contained in a medical thermometer.
"Tsalkiy" - without him, our bones would be fragile and fragile.
"Roshof" - a substance consisting of atoms of this element, was smeared with the hair of the dog of the Baskervilles.
Teacher. If you easily figured out the anagram words, tell yourself: "I am great!"
Chemical signs and chemical formulas
(Homework check)
Individual work at the blackboard using cards.
Digital dictation
Pupils exercise control over the implementation of the dictation by the method of mutual verification.
Exercise. Opposite the correct statements, put the number 1, opposite the incorrect ones - 0.
1. A chemical element is a certain kind of atoms.
2. In each cell of DI Mendeleev's table, in addition to the designation and name of the element, two numbers are written: the upper one is the relative atomic mass of the element, the lower one is its ordinal number.
3. The chemical element gallium was named after France.
4. In DI Mendeleev's table, the elements are arranged, as a rule, in descending order of their atomic masses.
5. Values of the relative atomic mass and the mass of the atom, expressed in a. i.e., they never match numerically.
6. Simple substances are called substances consisting of atoms of one element.
8. Mass fraction of an element shows what part (fraction) is the mass of this element from the total mass of the substance.
9. The relative molecular weight of water H 2 O is equal to 20.
10. Mass fraction of calcium in calcium oxide CaO is 71%.
Rule answers: 1 - 1, 2 - 0, 3 - 1, 4 - 0, 5 - 0, 6 - 1, 7 - 0, 8 - 1, 9 - 0, 10 - 1.
Operational and executive stage
Teacher. You know that the chemical formulas of substances show the quantitative ratios in which atoms are connected to each other, you also learned how to calculate the mass fraction of an element by the chemical formula of a substance. For example, in water H 2 O there are two hydrogen atoms per oxygen atom, or 11% N and 89% O. In carbon dioxide CO 2 there are two oxygen atoms per carbon atom.(demonstration of models of molecules of these substances.)
Valence
Teacher. Valence is the ability of atoms to attach to themselves a certain number of other atoms.
One atom of a monovalent element combines one atom of another monovalent element (HF, NaCl) ... With an atom of a divalent element, two atoms of a monovalent(H 2 O) or one atom of bivalent(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 connect to.
Rules for determining valency
elements in connections
The valency of hydrogen is taken as I (unit). Then, in accordance with the formula of water H 2 O, two hydrogen atoms are attached to one oxygen atom.
Oxygen in its compounds always exhibits valency II. Therefore, carbon in the CO 2 ( carbon dioxide) has valency IV.
Teacher.How to determine the valency of an element based on the table of D.I. Mendeleev?
For metals in groups a, the valency is equal to the group number.
In non-metals, two valencies are mainly manifested: the highest and the lowest (diagram).
The highest valency is equal to the group number.
The 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.
Teacher.For example: sulfur has the highest valency VI and the lowest (8 - 6), equal to II; phosphorus exhibits valences V and III.
Valence can be constant (for elements of the main subgroups of Mendeleev's table) or variable (for elements of secondary subgroups in the table), but you will get acquainted with this phenomenon a little later, and if you are interested, read the 9th grade textbook.
The valence of the elements must be known in order to draw up the chemical formulas of the compounds. For this it is convenient to use the following table.
table
Algorithm for drawing up the formula for the compound P and O
Sequencing |
Formulation of phosphorus oxide |
1. Write element symbols |
|
2. Determine the valencies of the elements |
|
3. Find the least common multiple of the numerical values of the valencies |
|
4. Find the ratios between the atoms of the elements by dividing the found smallest multiple by the corresponding valencies of the elements |
10: 5 = 2, 10: 2 = 5; |
5. Write indices on element symbols |
|
6. Formula of the compound (oxide) |
Teacher. Remember two more rules for drawing up chemical formulas for compounds of non-metals with each other.
1) The element that is located in the table of D.I. Mendeleev to the right and above, and the highest valency is shown by the element located to the left and below. (Demonstration of D.I. Mendeleev's table.)
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 combination of silicon with carbon, the first exhibits the highest valence IV, and the second, the lowest IV. So the formula is SiC. It is silicon carbide, the basis of refractory and abrasive materials.
2) In the formulas of compounds, the non-metal atom exhibiting the lowest valence always comes second, and the name of such a compound ends in "id".
For example, CaO - calcium oxide, NaCl - sodium chloride, PbS - lead sulfide.
Now you yourself can write formulas for any compounds of metals with non-metals.
The text of the work is pre-written on the board. Two students solve the problem on the back of the board, the rest in notebooks.
Exercise 1.
Check if the formulas for the following compounds are written correctly: Na 2 S, KBr, Al 2 O 3,
Mg 3 N 2, MgO.
Task 2. Write the formulas for compounds of metals with non-metals: calcium with oxygen, aluminum with chlorine, sodium with phosphorus. Name these connections.
After completing the work, the students exchange notebooks, a mutual check takes place. The teacher can selectively check some notebooks, praise those students who did the fastest and made the least mistakes.
Consolidation of the studied material
Chatting with students about questions
1) What is valency?
2) Why is valence sometimes called the atomicity of an element?
3) What are the valences of hydrogen and oxygen?
4) What two valence values can non-metals exhibit?
5) How to determine the lowest and highest valence of non-metals?
6) How to find the smallest common multiple between the numerical values of the valencies?
7) Can atoms in a compound have free valencies?
8) Which of the two non-metals in the chemical formula of their compound takes 1st place, and which -
2nd? Explain on the example of oxide NO 2, using the table of D. I. Mendeleev.
Creative work in groups
Exercise. Using molecular model kits for various substances, construct molecular formulas and models for the following compounds:
1st group - copper and oxygen,
2nd group - zinc and chlorine,
3rd group - potassium and iodine,
4th group - magnesium and sulfur.
After completing the work, one student from the group reports on the completed assignment and, together with the class, provides an error analysis.
Home assignment. According to the textbook "Chemistry-8" by LS Guzei: § 3.1, tasks No. 3, 4, 5, p. 51. Those interested can prepare reports about the French scientist J.L. Proust and the English scientist J. Dalton.
Reflexive-evaluative stage and summing up the results of the lesson
Announce the grades for the lesson to the responding students, thank everyone for their work in the lesson. Assess the emotional state on a scale (see fig.). The teacher once again recalls the questions that need to be thought about in order to work effectively in the next lesson.
REFERENCE
Guzei L.S., Sorokin V.V., Surovtseva R.P. Chemistry-8, M .: Bustard, 2000; Tyldsepp A.A., Cork V.A. We study chemistry. M .: Education, 1988; R.V. Bukreeva, T.A. Bykanova Lessons from new technologies in chemistry. Voronezh, 1997.
§ 1 Valence of chemical elements
At one time, the composition of all substances was established on the basis of experimental data. However, it is possible to draw up chemical formulas without resorting to preliminary implementation of complex experiments that require long, painstaking work.
If we compare the formulas of such substances as water H2O, calcium oxide CaO, aluminum oxide Al2O3, carbon monoxide CO2, phosphorus oxide P2O5, sulfur oxide SO3 and chlorine oxide Cl2O7, then it can be seen that oxygen in all these compounds adds different the number of atoms of other chemical elements.
To determine the composition of binary or two-element, that is, compounds consisting of atoms of two chemical elements, and to compile their formulas, it is enough to know the valence of chemical elements.
Valence (from the Latin word Valentia - "strength") - the property of an atom of a chemical element to attach or replace a certain number of atoms of another chemical element
Since the atoms in a molecule are connected to each other by chemical bonds, the valence is determined by the number of simple (single) chemical bonds that a given atom forms with other atoms.
§ 2 Determination of valency by the formulas of compounds
How can this be imagined if one does not resort to the theory of the structure of the atom? Each atom has a certain number of potential chemical bonds - valence possibilities.
For example, hydrogen - one, oxygen and calcium - two each, aluminum - three, carbon - four, phosphorus - five, sulfur - six, chlorine - seven. These atoms can connect with each other only using these same valence possibilities.
Therefore, the atoms of chemical elements form compounds, obeying the law of constancy of composition.
The law of constancy of composition states that substances, regardless of whether they are found in nature or how they are obtained in the laboratory, always have the same composition.
The ability of elements to exhibit a particular valence value is determined by the structure of their atoms. Since the structure of atoms is usually studied later, we will learn how to determine the valency based on the position of the elements in the periodic table.
To do this, it should be borne in mind that each group (vertical column) of elements consists of two subgroups: main A and secondary B.
Metal elements located in the main subgroups of groups I and II exhibit a constant valency equal to the group number. The same applies to aluminum (group III). But the metal elements of group IV (main subgroup), tin and lead, are an exception and exhibit variable valence, numerically equal to 2 and 4. For many metals of secondary subgroups, the presence of variable valence is also characteristic, however, the highest valence value is usually equal to the group number!
Most of the non-metals, located in the main subgroups of groups from the fourth to the seventh, exhibit variable valence. Among the possible values of the valencies of non-metals, the highest and the lowest should be distinguished. The highest valency is equal to the group number, the lowest is the difference obtained by subtracting the number equal to the group number from the number 8. For example: the highest valence of the phosphorus element in group V is 5,
lowest: 8-5 = 3. Therefore, the valence of phosphorus is variable - 3 and 5. It should be remembered that the highest valence of non-metals is manifested only in compounds with oxygen, and the lowest - in compounds with metals and hydrogen. The valence of hydrogen is always 1 in all compounds, the valence of oxygen is always 2.
§ 3 Compilation of chemical formulas by valency
To draw up formulas for complex substances consisting of atoms of two non-metals, it should be borne in mind that the highest valency will be manifested by the element that is to the left or lower in the periodic system, and the lower one, respectively, by the one that is to the right or higher.
We compose formulas and names of substances by valency using the following algorithm:
1.we write down the signs of the elements (according to their presence) in the order: metal, hydrogen, non-metal, oxygen;
2. we arrange the values of the valencies of the elements according to the periodic system of chemical elements;
3. find the smallest common multiple of valency values (the smallest number that is divisible by both valency values), divide it by the valence of each element, obtain and write down the index;
4. we name the substance. We add the suffix id to the Latin root of the second element, indicate the Russian name of the first element and its valency, if it is not constant.
Let's compose the formula and name for a substance consisting of phosphorus and oxygen atoms:
1.we write down the signs P and O;
2. the highest valence of phosphorus, equal to 5, the valence of oxygen, as in all compounds, is equal to 2;
3.least common multiple of 10
10/5 = 2, we write the index at the sign P
10/2 = 5, we write the index at the O sign
it turned out P2O5;
4. Let's name the substance: the root of the Latin name for oxygen is "oxygenium" ox, to it we add the suffix id, we get the oxide. The Russian name for the first element is phosphorus, its valence is variable equal to 5. The name "phosphorus oxide 5" is obtained.
§ 4 Determination of the name of a substance by a chemical formula
Thus, when compiling the name of a substance that has a certain chemical formula, it is necessary to indicate the valency, and to indicate it, it is necessary to determine. According to the periodic table, this does not always work. You can determine the valency and compose the name of the substance using the algorithm:
1. indicate the valence of a known element;
2. multiply the indicated valency by the corresponding index;
3. the result obtained is divided by the index of the element with unknown valence;
4. we name the substance. We add the suffix id to the Latin root of the second element, indicate the Russian name of the first element and its valence.
Determine the valence and compose the name of the substance with the formula CrO3:
1. the oxygen valence is constant and equal to 2;
3.6/1 = 6. The valency of chromium is 6;
4.the name of the substance is chromium oxide 6.
Now we will learn how to make a formula by the name of the substance
1.we write down the signs of chemical elements in the right order;
2. we indicate the valencies, paying attention to the name. If the valence of the first element is variable, it will be specified. The valency of the second element is the lowest;
3. find the smallest common multiple of valency values (the smallest number that is divisible by both valency values), divide it by the valence of each element, obtain and write down the index.
Let's define the formula for sulfur oxide:
1.we write down the signs S and O.
2. The valence of sulfur is 4, the valence of oxygen, as in all compounds, is two.
3.least common multiple of 4
4/2 = 2, we write the index at the O sign
4/4 = 1, we write the index at the sign S;
4. it turned out SO2.
List of used literature:
- NOT. Kuznetsova. Chemistry. 8th grade. Textbook for educational institutions. - M. Ventana-Graf, 2012.
Images used:
Lesson topic: Drawing up chemical formulas of binary compounds by valence.
"The scientific study of subjects has two main or ultimate goals: foresight and benefit"
D. I. Mendeleev
Goals:
Educational: consider the concept of "valence" as the atomicity of an element, teach students to determine the valency in binary compounds, introduce students to different types of valency, repeat the concepts of a multiple of a given number, the least common multiple of several numbers, repeat the rule for finding the LCM of several numbers and the application of this rule; to draw the attention of students to the integration of chemistry and mathematics courses.
Developing: develop the cognitive interest of students, develop the ability to reason logically, apply previously acquired knowledge, competently express their thoughts.
Educational: to contribute to the fostering of interest in the subject, to the result of their labor, to develop the ability to work in pairs, collectively, to evaluate the answers of their comrades.
Planned learning outcomes:
Students should know:
definition of the concept of "valency";
valence of hydrogen and oxygen atoms in compounds.
Students should be able to:
determine the valence of atoms of other elements in binary compounds by the valence of hydrogen and oxygen atoms;
determine the valence of atoms of elements by the formulas of substances, using an algorithm for solving problems.
Basic concepts: valence, constant and variable valence, binary compounds, least common multiple.
Lesson type: combined.
Means of education: algorithm for determining valence.
Equipment: Periodic table of chemical elements of D. I. Mendeleev, ball-rod models of molecules, table "Algorithm for determining valence."
During the classes
Organizational stage: greeting students.
Updating basic knowledge.
Frontal conversation chemistry teacher with students on the topic "Chemical formula".
When studying chemistry, it is very important to learn how to compose formulas of chemicals.
What does the chemical formula express? (the composition of a certain substance taken in pure form)
Chemical formula- it symbol substance, atom, molecule, ion using symbols of elements, numerical and auxiliary signs.
By the chemical formula, we can determine:
Type of substance,
Qualitative and quantitative composition,
Relative molecular weight
Mass fraction of a chemical element in a given substance,
Valence of chemical elements.
All substances are made up of atoms. One of the main properties of atoms is the ability to form chemical bonds. Atoms of different elements can form a certain number of bonds characteristic of them.
Let's compare the qualitative and quantitative composition in molecules: HCl, H 2 O, NH 3, CH 4.
What do molecules have in common? (presence of hydrogen atoms)
How do these substances differ from each other? (these substances have a different number of hydrogen atoms)
A hydrogen atom cannot attach more than one atom of another chemical element, therefore the hydrogen valence is taken as one. And therefore, the valence of all other elements is compared with the valence of hydrogen.
Examples:
HCl - one chlorine atom is bonded to one hydrogen atom;
H 2 O - one oxygen atom binds two hydrogen atoms;
NH 3 - one nitrogen atom binds three hydrogen atoms;
CH 4 - one carbon atom links four hydrogen atoms.
Why do different atoms hold different numbers of hydrogen atoms?
( each atom forms a certain number of bonds with other atoms).
This is called valence.
Valence- this is the property of atoms to hold a certain number of other atoms in the compound.
Valence is indicated by Roman numerals.
Notes on the board and in notebooks:
| I I | I II | I III | I IV |
The valency of the hydrogen atom is taken as one - I, and for oxygen - II.
Student message about valence.
At the beginning of the 19th century, J. Dalton formulated the law of multiple ratios, from which it followed that each atom of one element can combine with one, two, three, etc. atoms of another element (as, for example, in the considered compounds of atoms with hydrogen).
In the middle of the 19th century, when the exact relative weights of atoms were determined (I. Ya.Berzelius and others), it became clear that greatest number atoms with which a given atom can be combined does not exceed a certain value depending on its nature. This ability to bind or replace a certain number of other atoms and was named by E. Frankland in 1853 "valence".
Since at that time no compounds were known for hydrogen where it would be bonded to more than one atom of any other element, the hydrogen atom was chosen as the standard having a valency equal to 1.
In the late 50s. XIX century A. S. Cooper and A. Kekule postulated the principle of constant tetravalence of carbon in organic compounds. The concept of valence formed an important part of A.M.Butlerov's theory of chemical structure in 1861.
Periodic law of D.I. Mendeleev in 1869 revealed the dependence of the valence of an element on its position in the periodic system.
V. Kossel, A. Werner, G. Lewis contributed to the evolution of the concept of “valence” in different years.
Since the 30s. XX century, ideas about the nature and nature of valence were constantly expanding and deepening. Significant progress was achieved in 1927, when W. Geitler and F. London performed the first quantitative quantum-chemical calculation of the hydrogen molecule H 2.
Conversation with students:What is valency?
The definition of valency in different sources sounds differently. Let's think about which of these three definitions is more perfect and what are the disadvantages of the others.
1st row
"Valence
chemical element - the ability of its atoms to combine with other atoms in certain proportions. "
2nd row
"Valence
- the ability of atoms of one element to attach a certain number of atoms of another element. "
3rd row
"Valence
- the property of atoms, entering into chemical compounds, to give or receive a certain number of electrons or to combine electrons to form common electron pairs for two atoms. "
Group discussion, we come to the conclusion that the 3rd definition most accurately reflects the essence of the definition of valency.
Presentation of new material:
The topic of our lesson: Drawing up formulas of binary compounds by valency.
New material is easier to remember if it is strung on the already acquired knowledge. Therefore, we now have to work with the extraction of this material from memory. And the math teacher will help you with this.
The teacher conducts a dialogue with the students:
Teacher: list multiple numbers that are multiples of 12.
12, 24, 36, 48 ….
Teacher: list some numbers that are multiples of 18.
18, 36, 54, 72……
Teacher: What are the numbers that are multiples of both 12 and 18.
36 and 72, etc.
Teacher: what is the least common multiple of 12 and 18.
Teacher: Formulate the definition of the least common multiple of multiple numbers.
The least common multiple of several numbers is the smallest natural number that is divisible by each of the given numbers.
Teacher: formulate a rule for finding the least common multiple of two or more numbers.
To find the LCM of two or more numbers, it is necessary to decompose these numbers into prime factors, then, taking the decomposition of one of them, multiply it by the missing prime factors from the expansions of other numbers.
The teacher suggests the following exercise:
№1 ... Find A) LCM (48, 90); B) LCM (6, 15, 20)
Solution:
A) 48/2 90/2 B) 6/2 15/3 20/2
24/2 45/3 3/3 5/5 10/2
12/2 15/3 1/ 1/ 5\5
LCM (48.90) = 2 4 * 3 * 3 * 5 LCM (6.15.20) = 2 3 * 5 * 3
LCM (48.90) = 720 LCM (6.15.20) = 60
Answer: A) 720; B) 60.
Teacher: what is the LCM of two coprime numbers?
The LCM of two coprime numbers is equal to their product.
№2. Find the LCM (7.11)
Teacher: As you remember, there is another way to find the LCM of numbers - this is a selection method.
Find the LCM following numbers selection method:
A) 10 and 2 B) 14 and 21 C) 20 and 15 D) 2; 3 and 5
Answers: A) 10 B) 42 C) 60 D) 30.
Teacher: Guys, we repeated how you can find the LCM of two or more numbers. And now you will get acquainted with how this knowledge is applied in chemistry to determine valence.
W. Goethe once said: “It’s just not enough to know everything, you need to be able to use knowledge”.
Determination of the valence of elements by formulas in binary compounds.
Remember: in the formulas of binary compounds, the number of valence units of all atoms of one element is equal to the number of valence units of all atoms of another element.
Write down the formula of the substance. R 2 O 5
Indicate the valence of one of the elements in Roman numerals. R 2 O 5 (II)
Determine the valence of another chemical element. R 2 O 5
X * 2 = II * 5 X = V
Compilation of chemical formulas of binary compounds by the valence of elements.
Writing characters chemical elements that make up the formula, putting down the corresponding valency values over them:
CaO, B 2 O 3, CO 2,
We draw up a formula valency compounds:
a) CaO: if the valences of chemical elements are equal, then we do not put the indices.
b) In 2 O 3: if the values of the valencies are not divisible by each other, we put the values of the valencies in a cross.
c) CO 2: if the valence of one element is divided by the valence of another, then the value of the higher valency is divided by the value of the smaller one and the resulting number in the form of an index is placed next to the element with a lower valence.
The exercise: determine the valence of elements in substances: students go to the blackboard in a chain. We project the task onto the board.
SiH 4, CrO 3, H 2 S, CO 2, CO, SO 3, SO 2, Fe 2 O 3, FeO, HCl, HBr, Cl 2 O 5, Cl 2 O 7, PH 3, K 2 O, Al 2 O 3, P 2 O 5, NO 2, N 2 O 5, Cr 2 O 3, SiO 2, B 2 O 3, SiH 4, Mn 2 O 7, MnO, CuO, N 2 O 3.
Evaluative and reflective stage.
Primary check of knowledge assimilation. To complete this task, students receive the "Algorithm for determining valence" and tasks of three levels. Each student chooses their level of assignment.
Reproductive level (“3”). Determine the valence of atoms of chemical elements by the formulas of the compounds: NH 3, Au 2 O 3, SiH 4, CuO.
Application level (“4”). From the given series, write out only those formulas in which metal atoms are divalent: MnO, Fe 2 O 3, CrO 3, CuO, K 2 O, CaH 2.
Creative level (“5”). Find a pattern in the sequence of formulas: N 2 O, NO, N 2 O 3 and put the valencies over each element.
| Algorithm for determining valence | Example |
|
| 1. Write down the formula of the substance. | ||
| 2. Designate the known valence of the element | ||
| 3. Find the number of valence units of atoms of a known element by multiplying the valence of the element by the number of its atoms | ||
| 4. Divide the number of units of valence of atoms by the number of atoms of another element. The received answer is the desired valence | 2 | 2 |
| 5. Make a check, that is, count the number of valence units of each element | I II | I II |
We carry out a mutual check of the completed task(students exchange notebooks).
Correcting bugs: o answers on the board.
Summing up the lesson.
Conversation with students:
What problem did we pose at the beginning of the lesson?
What conclusion have we come to?
Give a definition of "valency".
How to determine the LCM?
What is the valence of a hydrogen atom? Oxygen?
How to determine the valence of an atom in a compound?
Assessment of student work.
Homework: textbook G.E. Rudzitis, F.G. Feldman §17, p. 60, ex. 1, 2, 4, 6
Problematic question: Why hydrogen and lithium have constant valence, but helium has no valence?
"How to succeed for students is to catch up with those who are in front, and not wait for those who are behind."
Aristotle
We wish you to always be ahead.
