The word "atom" comes from the Greek "atomos", meaning uncuttable or indivisible. An atom is the smallest particle of an element that retains the chemical properties of that element. Atomic Structure, deal with the Structure of atoms, electronic configuration, valency and more. Atoms consist of three fundamental particles: protons, electrons, and neutrons. The nucleus (centre) of the atom contains the protons (positively charged) and the neutrons (no charge). The outermost regions of the atom are called electron shells and contain the electrons (negatively charged). Atom is divided into two parts – Atomic Structure and extra nucleus part. Atomic Structure consists of a positively charged proton (P+) and neutral neutron. Extra nucleus part contains negatively charged electrons (e-). Atoms have mass as all elements and compounds have mass. Mass Number: Sum of the number of protons and neutrons present in the nucleus of an atom is known as the mass number (A). The chapter also deals with, how notation of element is done, and it's electronic configuration, the arrangement of electrons in the shells.