An atom is the smallest particle of a chemical element possessing the chemical properties of the element. The first model of an atom was given by J.J. Thompson; his model explained an atom as a sphere of positively charged matter in which electrons were embedded. This model could not explain all features of optical spectra of hydrogen and other elements. In 1911, Rutherford established that an atom consist of a positively charged nucleus and a number of negatively charged electrons which revolve around the nucleus in various orbits. The nucleus consist of a number of neutral particles called neutrons and a number of positively charged particles called protons.