This book has been designed to be a textbook for diploma first year students of MSBTE. It consists of four modules. Our world is made up of atoms and our first module provides information of atomic structure. In modern age, the scientists like Thomson, Rutherford, Bohr, Dalton etc. worked on atom and revealed the nature of the atom. This module also contain study of fundamental particles of atom, atomic number, atomic mass number, isotopes, isobar, valency, ionic and covalent bond and many more.