"Chemical Formula and Valence" Water in Nature PPT Courseware 6

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"Chemical Formula and Valence" Water in Nature PPT Courseware 6

2. Compound price

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

An atomic group is a group of atoms that often acts as a whole in compounds and chemical reactions, also called roots.

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.

practise

Try to write the chemical formula of the following substances

It is known that Na has a valence of +1 and O has a valence of -2. Write their chemical formulas.

It is known that Fe has a valence of +3 and O has a valence of -2. Write their chemical formulas.

It is known that Al has a valence of +3 and Cl has a valence of -1. Write their chemical formulas.

It is known that Mg has a valence of +2 and O has a valence of -2. Write their chemical formulas.

Cross method: When the absolute values ​​of positive and negative valencies of two elements in a compound are not equal, the ratio of the number of atoms is opposite to the absolute value of their valencies.

1. Write the symbol P O

2. Standardized valence P O

3. The absolute value of the chemical valence is placed crosswise in the lower right corner of the element symbol P2 O5

4. Simplify

5. Check

To write a chemical formula based on the valence of elements, follow the following steps:

Two prices are listed in a row. The prices are crossed and written on the lower right.

Reduce the price to a dime. Check if the total price is zero.

practise:

It is known that Al has a valence of +3 and O has a valence of -2. Write their chemical formulas.

It is known that C has a valence of +4 and O has a valence of -2. Write their chemical formulas.

It is known that Ca has a valence of +2 and OH has a valence of -1. Write their chemical formulas.

It is known that Mg has a valence of +2 and SO4 has a valence of -2. Write their chemical formulas.

Three: Calculate the valence based on the chemical formula

Example (2): Given that O has a valence of -2, find the valence of the iron element in Fe2O3.

Solution: Let the valency of iron element in Fe2O3 be X

2X+(-2)×3=0

X=+3

Exercise: Given that SO4 has a valence of -2, find the valence of the iron element in FeSO4.

The valency of variable valence elements must be determined based on calculations.

Four: How to read chemical formulas: read from right to left

A substance composed of two elements is read as "a certain chemical"

Such as: CaCl2 MgO

A substance composed of non-metallic elements and oxygen elements, pronounced as "a few oxides a few"

Such as: P2O5 SO2

A substance composed of metallic elements and atomic groups, pronounced as "a certain acid"

Such as: CaCO 3 CuSO4 NaNO 3

A substance composed of hydrogen atoms and atomic groups is pronounced "a certain acid".

Such as: H2CO3 H2SO4

A substance composed of metallic elements and hydroxyl radicals, pronounced as “hydrogen oxide”

Such as: NaOH Ca(OH) 2 Cu(OH) 2 Fe(OH) 2

Notice:

When it contains "Fe" or "Cu", the valence must be marked before naming to determine whether to add "sub".

Such as: Fe +2 +3

Cu +1 +2

When the price is high, it is read as "iron or copper";

At low prices it is read as "ferrous iron or copper".

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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