The smallest unit of matter is an atom. It has the properties of an element. An atom comprises of a dense core called nucleus surrounded by a series of outer shells. The electrons are present in these shells. The nucleus contains the protons and neutrons. Protons have a positive charge, while the neutrons are neutral. Two or more atoms tightly bound together form a molecule. The molecules made up of two atoms are known as diatomic. Oxygen, nitrogen, hydrogen, iodine are diatomic molecules. Earth’s atmosphere is comprised mainly of the diatomic molecules. A molecule is the smallest part of a compound. Students will learn the laws of chemical combination, atoms and molecules. They will get to know how to write a chemical formula, molecular mass and mole concepts and some numerical problems related to these concepts.