This chapter, "Atoms And Molecules", tells you about the idea of divisibility of matter. On dividing the matter to such an extent that it can't divide further, the particle obtained is called an atom. There are a lot of scientists who worked in this field and gave a lot of theories. Some of the universally accepted theories are the law of conservation of mass, i.e. the total weight of the species reacting and the total weight of the products remains constant. If the compounds are of pure origin, both physically and chemically, then the compound has fixed or definite proportions. The above-given theory is commonly known as the law of definite or fixed proportions.
A molecule of a compound or an element is the smallest particle that can have independent existence under the usual conditions. It can exhibit all the physical and chemical characteristics of the element of the compound. The chemical compound's formula can be defined as the representation of the number of atoms of every combining element. Thus the chemical formula of a compound projects its adjoining elements and the number of atoms present in each element. The polyatomic ion can be defined as the collection of atoms that act as an ion (have a charge). The chemical formula of the molecular compound shows the valency of every element. There is an interesting thing in the ionic compound that the charge on it helps us determine the compound's chemical formula. A great scientist named Avogadro derived a special number that has given a great boost to Analytical Chemistry. He derived it as the number of atoms in exactly 12g of carbon-12 is accurately 6.02214076 x 1023.