Atoms are the basic building blocks of matter whereas molecules contain one or more atoms. Atoms are the smallest unit that contains a chemical element. We will learn in this chapter about the law of conservation and the law of constant/definite proportions. Further in the chapter, we will discover the atomicity of the molecule. It is classified as monatomic, diatomic, triatomic, tetra-atomic, or polyatomic.

The Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. In other words, we will learn that the mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction.

This law was stated by Proust as “In a chemical substance the elements are always present in definite proportions by mass”. The Law of constant composition states that a given chemical compound always contains its component elements in a fixed ratio (by mass) and does not depend on its source and method of preparation.

Compounds are made by the combination of 2 or more kinds of; the atoms in a compound are chemically bonded and hence cannot be separated at the easy ratio of the atomic mass of atoms in any substance like water is the same in all situations.

A molecule is made of a combination of atoms. Each element has a specific number of atoms that combine together and form a molecule. We will also learn about the atomicity of the molecule. They are monoatomic, diatomic, triatomic, tetra-atomic, or polyatomic.