Notes on Valency, Molecular Formula and Chemical Reaction | Grade 8 > Science > Matter | KULLABS.COM

• Note
• Things to remember
• Videos
• Exercise
• Quiz

#### Valency

The combining capacity of elements or the radicals with the other elements or radicals to form the molecule of an element or compound is called valency. Numbers such as 1, 2, 3, 4, 5, and 6 represents the valency.

#### Valence shell and Valence electrons

The outermost orbit or shell of an atom is called valence shell and the number of electrons present in the valence shell of an atom is called valence electrons.

#### Duplet and Octet

The arrangement of two electrons in the K-shell is said to be duplet. It is chemically inert because it has a complete number of electrons in its K-shell.

Except helium, the other inert gasses have 8 electrons in their valence shell. Such arrange of the stable group of 8 electrons in their valence shell is said to be octet.

#### Molecular formula

The molecular formula is defined as the symbolic representation of the molecule of a substance.

The molecular formula of element: The molecular formula of an element is defined as the symbolic representation which shows the actual number of atoms in one molecule of the element.

The molecular formula of the compound: The molecular formula of a compound is defined as the symbolic representation which shows the actual number of an atom of different elements present in one molecule of the compound.

#### Chemical reaction

The process by which a chemical change takes place is called as a chemical reaction. Those substances that take part in a chemical reaction are called reactants. Those substances which are formed by a chemical reaction are called products.

##### Word equation

The chemical reaction which is expressed in terms of full names of reactant and product molecules is called word equation. For example, Hydrogen reacts with oxygen to produce water.

Hydrogen + oxygenWater

(Reactants) (Products)

##### Formula equation

The chemical equation which is written in terms of symbols by writing their molecular formula is called formula equation.
Heat Potassium chlorate → Potassium chloride + oxygen
KClO3 → KCl +O2

##### Ways to write chemical reaction

Step 1: Write the word equation.

Heat Potassium chlorate → Potassium chloride + oxygen

Step 2: Convert the word equation into formula equation.

KClO3 → KCl +O2

Step 3: Count the number of atoms, ions, etc of the reactant and product and make them equal by using the suitable number in front of them.

2KClO3 → 2KCl +3O2

• The combining capacity of elements or the radicals with the other elements or radicals to form molecules of an element or compound is called valency.
• The arrangement of two electrons in the K-shell is said to be duplet.
• The molecular formula is defined as the symbolic representation of the molecule of a substance.
• The process by which a chemical change takes place is called as the chemical reaction.
• The chemical reaction which is expressed in terms of full names of reactant and product molecules is called word equation.
• The chemical equation which is written in terms of symbols by writing their molecular formula is called formula equation.
.

### Very Short Questions

The combining capacity of elements or the radicals with the other elements or radicals to form the molecule of an element or compound is called valency. Natural numbers such as 1, 2, 3, 4, 5, and 6 represents the valency.

The outermost orbit or shell of an atom is called valence shell and the number of electrons present in the valence shell of an atom is called valence electrons.

The molecular formula of an element is defined as the symbolic representation which shows the actual number of atoms in one molecule of the element.

The molecular formula of a compound is defined as the symbolic representation which shows the actual number of an atom of different elements present in one molecule of the compound.

The chemical reaction which is expressed in terms of full names of reactant and product molecules is called word equation. For example, Hydrogen reacts with oxygen to produce water. In word equation, we have

Hydrogen + oxygen→Water

The chemical equation which is written in terms of symbols by writing their molecular formula is called formula equation.
Heat Potassium chlorate → Potassium chloride + oxygen
KClO$$\overset{Heat}{\rightarrow}$$ KCl +O2

Following are the ways to write chemical equation:

When potassium chlorate is heated potassium chloride and oxygen is formed. Writing in chemical equation, we have

Step 1: Write the word equation.

Potassium chlorate  $$\overset{Heat}{\rightarrow}$$Potassium chloride + oxygen

Step 2: Convert the word equation into formula equation.

KClO3  $$\overset{Heat}{\rightarrow}$$KCl +O2

Step 3: Count the number of atoms, ions, etc of the reactant and product and make them equal by using the suitable number in front of them.

2KClO3  $$\overset{Heat}{\rightarrow}$$ 2KCl +3O2

0%
• ### The combining capacity of elements with the other elements to form molecule of an element or compound is called ______.

Molecular formula
Molecular weight
Valency
• ### ______ is defined as the symbolic representation of the molecule of a substance.

Molecular weight
Skeletal Formula
Balanced Formula
Molecular formula

Reactors
Products
Indicators
Reactants

Residue
Result
Products
Reactants
• ### The chemical reaction which is expressed in terms of full names of reactant and product molecules is called _____.

Formula equation
Skeletal equation
Balanced equation
Word equation
• ### The chemical equation which is written in terms of symbols by writing their molecular formula is called ______.

Skeletal equation
Formula equation
Balanced equation
Word equation
• ### The number of electrons present in the valence shell of an atom are called ______.

Outermost electron
Last electron
Leveled electron
Valence electron
• ### The outermost orbit or shell of an atom is called ______.

Innermost shell
Special shell
Last shell
Valence shell

## ASK ANY QUESTION ON Valency, Molecular Formula and Chemical Reaction

Forum Time Replies Report
##### shivam parliwal

how to takeout atomic mass