Molecules

Question 1 What is a molecule. Give example?

Question 2 How do atoms usually exist?

Question 3 Why atoms form molecules or ions?

Question 4 Define covalent bond?

Question 5 What is meant by molecule of an element?

Question 6 What is meant by molecule of a compound?

Question 7 Define the term atomicity?

Question 8 What are monoatomic molecules.Give example?

Question 9 What are diatomic molecules.Give example?

Question 10 What are triatomic molecules.Give example?

Question 11 What are tetra atomic molecules.Give example?

How do Atoms Exist

Atoms of only few elements called noble gases are chemically unreactive and exist in free state.Atoms of most of the elements are chemically reactive and do not exist in the free state.

Atoms usually exist in two ways:

1)In the form of molecules

2)In the form of ions

When atoms form molecules or ions,they become stable(because in doing so they acquire the stable electron arrangement of noble gases)

Molecules
A molecule is an electrically neutral group of two or more atoms chemically bonded together.

The forces which hold atoms together in a molecule are called covalent bond.

Molecules can exist in free state because they are very stable.
Molecules can be formed by atoms of the same element or of different elements.Depending on this there are two type of molecules:

1)Molecules of elements

2)Molecules of compounds

Molecules of Elements
The molecule of an element contains two or more similar atoms chemically combined together.

For Ex:A molecule of hydrogen contains two hydrogen atoms combined together.
A molecule of oxygen contains two oxygen atoms combined together.
A molecule of ozone contains three oxygen atoms combined together.
A molecule of phosphorus contains four phosphorus atoms combined together.

The number of atoms present in a molecule of an element is called its ATOMICITY.

1)Noble gases have one atom each in their molecules,so the atomicity of noble gases is 1.
The atomicity of metal elements like sodium,magnesium,aluminum,copper,iron etc is taken as 1.
They are called as Monoatomic molecules.

2)Hydrogen,Nitrogen,Oxygen,Chlorine,bromine etc have two atoms in their molecules.So the atomicity is 2.They are called as diatomic molecules.

3)Ozone has 3 atoms in its molecules,so the atomicity is 3.They are called as triatomic molecules.

4)Phosphorus has 4 atoms in its molecules,so the atomicity is 4.they are called as tetra-atomic molecules.

Molecules of Compounds
The molecule of a compound contains two or more different types of atoms chemically combined together.

For Ex: HCl,water,carbon dioxide,ammonia,methane etc.