Valency - and Finding Chemical Formula using it

What is Valency?

The combining power (or capacity) of an element is known as its valency.

Picturize an octopus holding hands with humans.

One octopus has 8 arms and one human has 2 hands.

So, 1 octopus has hold hands with 4 humans = 2 hands x 4 humans = 8 arms of octopus

We can say;

• The ‘combining capacity’ of octopus is 8
• And that of humans is 2

So the combination of arms of Octopus to Human may be written as;
Octopus : Human

8 : 2

4 : 1

What is a Molecule?

A group of atoms chemically bonded together is called Molecule.

Combining capacity of elements is called valency

Here the ‘combining capacity’ of octopus is 8 and each human is 2

Here are the valency of a few elements:

How is valency used to write the formula of compounds?

What is a chemical formula?

A chemical formula is a symbolic representation of a chemical compound.

It tells us about the type and number of atoms present in the compound.

For example, the chemical formula of salt is NaCl. 

It tells us that NaCl is composed of one atom of Sodium (Na) and 1 atom of Chlorine (Cl)

What is a Molecule?

A group of atoms chemically bonded together is Here are the valency of a few elements: called Molecule

Rules to follow while writing a chemical Formula

1. The valencies on the ion must balance
   
   
2. When a compound contains a metal and non metal.
   Name/Symbol of Metal is written first
   
   

Example 1

We write Calcium Oxide (CaO) and not Oxide Calcium

This is because Calcium (Ca) is a metal and Oxygen (O) is a non metal

Example 2

We write Sodium Chloride (NaCl) and not Chloride Sodium

What is a Molecule?

A group of atoms chemically bonded together is Here are the valency of a few elements:

called Molecule.

This is because Sodium (Na) is a metal and chlorine (Cl) is non metal 

3. In compounds formed with polyatomic ions (ions having more than one atom)
we write the polyatomic ion in a bracket and write the number of ions outside the bracket

Example 1
OH is a polyatomic ion having valency 1
Mg has valency 2
They react to form Mg(OH) ₂
we write the polyatomic ion (in this case, OH) in a bracket and write the number of ions (in this case, 2) outside the bracket

What is a Molecule?

A group of atoms chemically bonded together is Here are the valency of a few elements:
called Molecule.

Example 1 - Water (H ₂ O)

It contains 2 elements: Hydrogen and Oxygen

Valency of Hydrogen is 1

Valency of Oxygen is 2

What is a Molecule?

A group of atoms chemically bonded together is Here are the valency of a few elements:
called Molecule.

Hence 2 atoms of Hydrogen shall combine with 1 atom of Oxygen
Hence,we get formula (H2O)

Example 2 - Hydrogen Chloride (HCl)
It contains 2 elements: Hydrogen and Chlorine
Valency of Hydrogen is 1
Valency of Chlorine is 1

These valencies interchange while writing formula as shown below

What is a Molecule?

A group of atoms chemically bonded together is Here are the valency of a few elements:
called Molecule.

Hence 1 atom of Hydrogen shall combine with 1 atom of chlorine
Hence,we get formula HCl

Example 3 - Hydrogen Sulphide (H ₂ S)
It contains 2 elements: Hydrogen and Sulphur
Valency of Hydrogen is 1
Valency of Sulphide is 2

These valencies interchange while writing formula as shown below

Hence 2 atoms of Hydrogen shall combine with 1 atom of Sulphur
Hence,we get formula H ₂ S

What is a Molecule?

A group of atoms chemically bonded together is Here are the valency of a few elements:
called Molecule.

Example 4 - Carbon Tetrachloride (CCl4)
It contains 2 elements: Carbon and Chlorine
Valency of Carbon is 4
Valency of Chlorine is 1

These valencies interchange while writing formula as shown below

Hence 1 atom of Carbon shall combine with 4 atoms of chlorine
Hence, we get formula CCl ₄

Note: Note the similarity with that of the example of Octopus and Humans holding hands. The Carbon (C) represents the Octopus and Chlorine (Cl) represents the Humans. Their arms form the bonds.

What is a Molecule?

A group of atoms chemically bonded together is Here are the valency of a few elements:
called Molecule.

Example -
In text Question - Page 39 Q1,Q2,Q3
NCERT Question - Q4,5