All substances are made up fine particles. You have learnt that a particle of a substance contains all its properties. Such a substance can be an element or a compound. A small iron filing is made up of fine particles of iron. A piece of chalk is made up of fine particles that contain all the properties of chalk. A particle of common salt contains all the properties of the common salt (compound).
Suppose we try to divide a glass of water into two equal parts of water. Then we again divide each half into two equal parts. If we could go on getting smaller quantities of water in this way, we would eventually come to one tiny drop, so small that we cannot see it with a microscope (or magnifying glass). Such a small particle of water retains all the properties of water (compound). When a particle of water is so tiny that a further division of the particle would change its properties, we call it a molecule.
A molecule is defined as the smallest unit of an element or a compound which can exist independently.
An atom is the smallest possible particle of the element; an atom does not exist alone.
All atoms of an element are alike. The atoms of the element hydrogen are all alike but they differ from the atoms of the element oxygen, or from the atoms of other elements. The building units of all substances are atoms.
A molecule which consists of atoms of two or more different elements is called the molecule of a compound. All molecules of a compound or an element are alike. For example, all molecules of water are alike. But molecules different compounds are not identical; they are different.