‘Poly' means many 

‘Atomic' means atoms

‘Ion' means atom carrying charged

Group of atoms containing charge is called Polyatomic Ion.

Examples of Polyatomic Ions-Teachoo-01.jpg

Examples of Polyatomic Ions

  • Ammonium - NH 4 +

 

  • Hydroxide - OH -

 

  • Nitrate - NO 3 -

 

  • Phosphate - PO 4 3-

 

  • Sulphate - SO 4 2-

 

What is a Molecule?

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

called Molecule.

 

How to write Formula of compounds formed with Polyatomic Ions

The formula of Polyatomic ion is written in brackets.

Example - In Aluminium Sulphate Al 2 (SO 4 )3

SO 4 2- is a polyatomic ion consisting of Sulphur (S) and Oxygen (O)
The subscript ‘3’ in Al2(SO 4 ) 3 applies to both S and O
And so, SO 4 is put in brackets with subscript ‘3’ outside the brackets.
On the other hand, Al does not have brackets as it is monoatomic.

 


Example - Magnesium Hydroxide Mg(OH)2

  • Valency of Magnesium is 2
  • Charge on Magnesium is 2+
  • Charge on Hydroxide 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 Formula of Magnesium hydroxide is Mg(OH) 2

 

Note - Hydroxide is written in brackets as 2 is applicable to O and H which means there are 2 OH - ions (2 O and 2 H atoms)

 

Example - Sodium Hydroxide NaOH

  • Valency of Sodium is 1
  • Charge on Sodium is 1+
  • Charge on Hydroxide 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 Formula of Sodium hydroxide is NaOH

  1. Class 9
  2. Chapter 3 Class 9 - Atoms And Molecules (Term 2)

About the Author

CA Maninder Singh's photo - Founder at Teachoo
CA Maninder Singh
CA Maninder Singh is a Chartered Accountant for the past 11 years and a teacher from the past 11 years. He teaches Science, Accounts and English at Teachoo