Atomicity is the no.of atoms present in a molecule.On the basis of atomicity,molecules can be classified as:
1) MONOATOMIC - only one atom. e.g.He,Ne,Ar,Xe,Fe,Al etc.
2) DIATOMIC - two atoms. e.g.Hydrogen,Nitrogen,Oxygen etc.
3)TRIATOMIC - three atoms. e.g.Ozon,Carbon -di-oxide,Nitrogen-di-oxide etc.
4)TERA-ATOMIC - four atoms. e.g.Phosphorus,Hydrogen peroxide etc.
5) POLYATOMIC - more than four atoms . e.g.Methane (penta-atomic) etc.

Atomicity:
Atomicity refers to the state of being composed of individual atoms. It describes the number of atoms that make up a molecule. A molecule can be classified as atomic, diatomic, triatomic, tetratomic, pentatomic, hexatomic, heptatomic, octatomic, and so on, depending on the number of atoms it contains.

Tetratomic Molecule:
A tetratomic molecule is composed of four atoms. One example of a tetratomic molecule is methane (CH4). Methane consists of one carbon atom bonded to four hydrogen atoms. The carbon atom is located at the center of the molecule, with the four hydrogen atoms arranged symmetrically around it. Methane is a stable and abundant gas that is the main component of natural gas. It is widely used as a fuel and is also produced by various natural processes, including the decomposition of organic matter.

Octa-atomic Molecule:
An octa-atomic molecule is composed of eight atoms. One example of an octa-atomic molecule is sulfuric acid (H2SO4). Sulfuric acid is a strong acid commonly used in various industries. It consists of two hydrogen atoms, one sulfur atom, and four oxygen atoms. The sulfur atom is bonded to the four oxygen atoms, and each oxygen atom is also bonded to a hydrogen atom. The arrangement of atoms in sulfuric acid gives it its acidic properties, making it corrosive and capable of reacting with many substances.

Summary:
- Atomicity refers to the number of atoms that make up a molecule.
- A tetratomic molecule contains four atoms, such as methane (CH4).
- An octa-atomic molecule contains eight atoms, such as sulfuric acid (H2SO4).
- Methane is a tetratomic molecule commonly found in natural gas and used as a fuel.
- Sulfuric acid is an octa-atomic molecule used widely as a strong acid in various industries.