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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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