Atom was considered as the smallest particle up to 19th century. A series of experiments were performed to explore the structure of atom. We shall discuss the structure of an atom and study the characteristics of atomic spectra and chemical properties of various atoms. An atom is made up of three subatomic particle electron, proton & neutrons. These three particle are called fundamental particle of matter. The size of nucleus of atom is very small in which neutron and protons are present so the entire mass of the atom is situated in nucleus. Protons and neutrons present in the nucleus are collectively termed an nucleons. Number of neutrons is called mass number of the element. The neutron and proton have approximately equal masses of 1 amu and the electron is about 1836 times lighter; its mass can sometimes be neglected as an approximation. The electron and proton have equal, but opposite, electric charges; the neutron is not charged. The electron and proton have equal, but opposite, electric charges; the neutron is not charged. The existence of electrons in atoms was first suggested, by J.J. Thomson, as a result of experimental work on the conduction of electricity through gases at low pressures, which produces cathode rays and x-rays, and a study of radioactivity by Becquerel, the Curies and Rutherford.