The existence of different kinds of matter is due to different atoms constituting them. Now the questions arise: What makes the atom of one element different from the atom of another element? Are atoms really indivisible, as proposed by Dalton, or are there smaller constituents inside the atom? We will learn about sub-atomic particles and the various models that have been proposed to explain how these particles are arranged within the atom. Credit for the discovery of electron and proton goes to J.J. Thomson and E. Goldstein, respectively. J.J. Thomson proposed that electrons are embedded in a positive sphere. Rutherford's alpha-particle scattering experiment led to the discovery of the atomic nucleus. Rutherford's model of the atom proposed that a very tiny nucleus is present inside the atom and electrons revolve around this nucleus. The stability of the atom could not be explained by this model. Niels Bohr’s model of the atom was more successful. He proposed that electrons are distributed in different shells with discrete energy around the nucleus. If the atomic shells are complete, then the atom will be stable and less reactive. J. Chadwick discovered presence of neutrons in the nucleus of an atom. So, the three sub-atomic particles of an atom are: electrons, protons and neutrons. Electrons are negatively charged, protons are positively charged and neutrons have no charges. The mass of an electron is about 0.0005 of the mass of a hydrogen atom. The mass of a proton and a neutron is taken as one unit each. Shells of an atom are designated as K, L, M, N, etc. Valency is the combining capacity of an atom. The atomic number of an element is the same as the number of protons in the nucleus of its atom. The mass number of an atom is equal to the number of nucleons in its nucleus. Isotopes are atoms of the same element, which have different mass numbers. Isobars are atoms having the same mass number but different atomic numbers. Elements are defined by the number of protons they possess.