"Chemical Formula and Valence" Water in Nature PPT Courseware 5

"Chemical Formula and Valence" Water in Nature PPT Courseware 5

1. Chemical formula

1. Definition: A formula that uses a combination of element symbols and numbers to express the composition of a substance is called a chemical formula.

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

③Does a chemical formula only represent one substance?

A chemical formula does not necessarily represent only one substance. For example, the chemical formula of red phosphorus is P, and the chemical formula of white phosphorus is also P.

④Can the chemical formula be written in any way?

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

The chemical formula can represent:

Macro

1. This substance.

2. The elemental composition of the substance.

microscopic

3. A molecule that constitutes a substance.

4. What kinds of atoms are made up of a molecule of this substance, and how many of each kind of atoms there are.

Note: Substances and elements use "composition"

Atoms and molecules are composed of

Number before element symbol:

Represents the number of atoms, molecules, or ions (can be changed):

The number in the lower right corner of the element symbol (cannot be changed):

Represents the number of atoms in each molecule.

Example: H2O SO2 CO2

The number in the upper right corner of the element symbol:

Represents the charge carried by ions.

Example: Ca2+ O2- Al3+

① Adding a number before an element symbol no longer represents a certain element and has no macroscopic meaning.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

② Adding a number before the chemical formula no longer represents the substance. It has no macroscopic significance and only represents n such molecules.

practice

1.What is the meaning of each of the following symbols?

(1) O oxygen element or an oxygen atom

(2) 2O 2 oxygen atoms

(3) CO2 means: carbon dioxide;

Carbon dioxide is composed of the elements oxygen and carbon;

a carbon dioxide molecule;

Each carbon dioxide molecule consists of 2 oxygen atoms and 1 carbon atom.

(4) 2CO2 2 carbon dioxide molecules

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

(1) 4 sulfur dioxide molecules

(2) 7 chlorine atoms

(3) 1 nitrogen molecule

(4) 5 sodium atoms

power enhanced

1. Among the meanings represented by the chemical formula "CO2", which one is wrong ( )

A represents the substance carbon dioxide

B represents a carbon dioxide molecule

C means that a carbon dioxide molecule is composed of one carbon element and two oxygen elements

D means carbon dioxide is composed of carbon and oxygen

2. Based on the meaning of "2" in the following chemical symbols, write the serial number that matches the meaning of the question on the horizontal line

①Ca2+ ②SO2 ③2H ④2HCl

⑴The numerical value of the charge carried by an ion is:

⑵Indicates the number of atoms

⑶Indicates the number of atoms contained in a molecule

⑷The number of molecules is represented by ____.

2. Compound price

1. Valence represents the number of mutual combinations between atoms. It is a property of elements when they form compounds.

Regulation:

(1) The chemical valence of H is +1

(2) The chemical valence of O is -2

(3) The algebraic sum of the valences of elements in a compound is 0

2. Valency rules:

(1) Compound prices can be divided into positive prices and negative prices

(2) Oxygen usually has a valence of -2

Hydrogen usually has a valence of +1.

(3) When a metal element is combined with a non-metal element, the metal element has a positive valence and the non-metal element has a negative valence.

(4) Some elements can show different valencies in different substances

(5) The algebraic sum of positive and negative valences in a compound is 0

(6) In a single substance molecule, the valence of the element is 0

power enhanced

1. It is known that in H3RO4, the hydrogen element has a valence of +1 and the oxygen has a valence of -2, then the valence of R is ( )

A, +3 B, +4 C, +5 D, +6

2. Among the following compounds, the nitrogen element has a valence of +5 ( )

A, N0 B, NO2 C, HNO2 D, KNO3

3. The chemical formula of the oxide of element R is R2O3, then the chemical formula of the substance composed of R element and OH- is ( )

A.ROH B.R(OH)2 C.R(OH)3 D.R(OH)4

4. Write the chemical formulas of the following substances.

Potassium oxide Silver oxide Barium oxide Aluminum oxide Ferrous oxide Iron oxide

Potassium chloride Silver chloride Barium chloride Aluminum chloride Ferrous chloride Ferric chloride Ammonium chloride

Potassium hydroxide Barium hydroxide Aluminum hydroxide Ferrous hydroxide Iron hydroxide

Potassium nitrate Silver nitrate Barium nitrate Aluminum nitrate Ferrous nitrate Ferric nitrate Ammonium nitrate

3. Calculation of relative molecular mass

1. Calculate relative molecular mass

Definition: The sum of the relative atomic masses of the atoms in a chemical formula.

Things to note when calculating relative molecular mass

1. Write chemical formulas correctly;

2. Accurately understand the meaning of the numbers in the chemical formula and before the formula;

3. Use a “+” sign between element symbols, and use a “×” sign between element symbols and the number of atoms.

2. Calculate the mass ratio of material constituent elements

The mass ratio of an element is the ratio of the relative atomic mass multiplied by the number of atoms. That is the total relative atomic mass ratio.

3. Calculate the mass fraction (also called percentage content) of an element in a substance

The mass fraction of an element in a substance can be expressed by the formula:

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