Molecule is the smallest particle of a compound. Atoms exist in free states in the form of molecule. 

• A molecule may be formed by the combination of two or more similar atoms of an element, such as oxygen molecule is formed by the combination of two oxygen atoms, molecule of hydrogen which is formed by the combination of two hydrogen atoms. 
• Molecules may be formed by the combination of atoms of two or more different elements. For example molecule of water. It is formed by the combination of two atoms of hydrogen and one atom of oxygen. Molecule of Nitric oxide or nitrogen monoxide. It is formed by the combination of one nitrogen atom and one oxygen atom. 
• A molecule takes part in chemical reaction. 

Most of the atoms exist in the form of molecule. Molecules are formed by the combination of two or more elements. 

Example: Molecule of hydrogen (H₂, Molecule of oxygen (O₂), Molecule of nitrogen (N₂), etc. 

• Molecules of elements 

• Molecules of Compounds