The molecule is the smallest particle which possesses the properties of a compound.

The 2n2 rule is a rule which is used to determine the number of electrons that can exist different shell of atoms. For example: for k shell (n)= 1
No of electron in k-shell = 2n2
= 2 (1)2
=2
No of electron in L-shell = 2n2
2.(2)2
2 x 4
=8

The tendency of an atom to have 8 electrons in their outermost shell is called octet. The tendency of an atom to have two electrons in their first shell is called duplet.

Those elements which can react easily with other elements during chemical reactions are called reactive elements.

Neon is an inert gas because its outermost orbit is fulfilled with 8 electron and it does not take part in chemical reaction.

The valency of oxygen is two because it gains two electrons during the chemical reaction.

The elements of group IA are called alkali metals because they easily react with water to produce alkali.

Chlorine is called reactive non-metals because it can easily gain one electron and possesses unit negative charge.