Polyatomic Ions

‘Poly' means many 

‘Atomic' means atoms

‘Ion' means atom carrying charged

Group of atoms containing charge is called Polyatomic Ion.

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