"Chemical Formula and Valence" Water in Nature PPT Courseware 4

Description

"Chemical Formula and Valence" Water in Nature PPT Courseware 4

review:

What is an element? What to express?

A general term for atoms of the same type with the same nuclear charge

represented by element symbols

What do the element symbols mean?

It can represent both an element and an atom of that element.

Ask students to use chemical symbols to represent the following substances:

1:Oxygen__________; Nitrogen__________

2:Carbon dioxide_______; sulfur dioxide_______

3:Water_______; Ferric oxide_______

4:Manganese dioxide________; Potassium chlorate________

5:Potassium permanganate____________

1. Chemical formula

1. Definition: A formula that represents the composition of matter using a combination of element symbols and numbers.

For example: the chemical formula of water is H2O

The chemical formula of oxygen is O2

The chemical formula of carbon dioxide is CO2

①Does any substance have a chemical formula?

Only pure substances can be represented by chemical formulas

②Can the same substance have different chemical formulas?

A substance can only be represented by one chemical formula

③Can the chemical formula be written in any way?

The writing of chemical formulas must be based on the results of experiments

2. Express meaning

Macroscopic meaning:

①Indicates a substance

②Indicates the elemental composition of a substance

Microscopic meaning:

③Represents a molecule of substance

④Indicates the composition of a molecule of a substance

What do the symbols H, 2H, H2, and 2H2 mean?

① H represents hydrogen element or a hydrogen atom.

② 2H represents 2 hydrogen atoms.

③ H2 represents the substance hydrogen; hydrogen is composed of hydrogen element; one hydrogen molecule; each hydrogen molecule is composed of 2 hydrogen atoms.

④ 2H2 represents two hydrogen molecules

summary

1. Adding a number before an element symbol no longer represents a certain element and has no macroscopic meaning. It only represents the number of atoms.

2. Adding a number before a chemical formula no longer represents the substance. It has no macroscopic meaning and only represents the number of molecules.

Practice. Fill in the blanks with element symbols or chemical formulas.

(1) 4 carbon dioxide molecules

(2) 7 iron atoms

(3) 1 oxygen molecule

(4) 5 sulfur atoms

(1) How to write and read chemical formulas of simple substances:

The chemical formulas of metal elements, solid non-metallic elements, and rare gases are customarily represented by element symbols; their names can be read directly. For example: Fe, C, He.

The chemical formulas of some non-metallic gases should indicate the number of atoms contained in the molecule in the lower right corner of the element symbol. For example: H2, O2.

(2) How to write and read chemical formulas of compounds:

Oxides are generally written with oxygen at the end, which is called "oxidation of something" or "how much oxidation of something". For example: MgO, CO2, Fe2O3

Compounds composed of metals and non-metals, metals are written on the left and non-metals are written on the right. It's called "something changes into something". For example: NaCl, K2S, CaO.

practise:

1. Among the following chemical formulas, the incorrect writing is ( )

(A) Calcium oxide (OCa) (B) Magnesium oxide (MgO)

(C) Ferric chloride (FeCl3) (D) Sulfur trioxide (SO3)

2. The pronunciation of the chemical formula Fe3O4 is ( )

(A) Iron trioxide tetraoxide (B) Iron trioxide tetraoxide

(C) Iron tetroxide (D) Iron oxide

2. Compound price

In chemistry, "valency" is used to represent the number of mutual combinations between atoms.

The valence of an element is a chemical property exhibited by the atoms of the element when forming compounds.

In order to easily determine the valency of elements in a compound, the following points need to be noted:

1. There are positive and negative price of chemical compounds.

(1) Oxygen usually has a valence of -2.

(2) Hydrogen element usually has a valence of +1.

(3) When metal elements are combined with non-metal elements, the metal elements show positive valence and the non-metal elements show negative valence.

(4) Some elements can have different valencies in different substances.

2. The algebraic sum of positive and negative valences in a compound is 0.

3. The valence of an element is a property displayed by the atoms of the element when forming a compound. Therefore, in a single substance molecule, the valence of an element is 0.

Relative molecular mass

Thinking: The mass of an atom is very small, so we use relative atomic mass. Then how can we express the mass of molecules composed of atoms, which is still very small?

The sum of the relative atomic masses of the atoms in the chemical formula is the relative molecular mass (symbol is Mr)

Use of relative molecules:

1. Calculate relative molecular mass based on chemical formula

Example 1. Find the relative molecular masses of O2, H2O and KMnO4

Exercise 1. Find the relative molecular masses of Fe3O4 and CO(NH2)2

Use of relative molecular mass

2. Calculate the mass ratio of the constituent elements of matter

The mass ratio between elements = the ratio of the sum of the relative atomic masses of each atom

Example 2. The mass ratio of manganese element and oxygen element in MnO2 is equal to

Exercise 2:

1. What is the mass ratio of each element in H2O2 and Cu(OH)2?

2. What is the mass ratio of each element in NH4NO3?

Class exercises

1. Explain the expressions of the following chemical elements

(1)H (2)2H (3)H2 (4)2H2 (5)H2O (6)2H2O (7)Mg2+ (8) Meaning of 2Mg2+

2. The mass of oxygen contained in 18gH2O is the same as the mass of oxygen contained in how many gCO2 ( )

A. 18g B. 22g C. 44g D. 88g

3. Heroin is a drug banned by the government. Its chemical formula is C21H23NO5. It is composed of __________ elements. There are ________ atoms in each heroin molecule.

4. It is known that the mass ratio of the three substances SO3, SO2 and O2 is 5:4:2, then the ratio of the number of molecules is __________

5. The chemical formula of a certain compound is HnROm, where the valence of R is ( )

A , +(m-n) B , +(2m-n) C , +(n-m) D , +(2n-m)

Summary of this topic

After studying this topic you should know:

1. The formula that uses element symbols to express the composition of matter is called a chemical formula.

2. The meaning of chemical formula:

Macro: represents a substance; represents the elemental composition of a substance.

Microscopic: represents a molecule; represents the composition of a molecule of a substance.

3. How to read and write chemical formulas

4. Valencies of some common elements and roots

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