Atoms are made up of subatomic particles such as Electrons, Protons and Neutrons. Of these protons and electrons are known as “Nucleons”,as the nucleus of the atom is made up of these particles. The positive nuclear charge is neutralized by the negatively charged electrons outside the nucleus. - - - + - - - - The British physicist Ernest Rutherford, winner of the 1908 Nobel Prize in chemistry, pioneered the field of nuclear physics with his experimental research and his development of the nuclear theory of atomic structure. The first atomic modal was proposed by J.J.Thomson, who discovered electrons in 1903. He however placed the electrons inside the nucleus.Such a modal could not account for many experimental observations. In 1911,Rutherford, based on his x-rays scattering experiment,proposed an alternate model & named it is “Planetary model or nuclear model” + + + + + + - + + + + In 1913,Danish scientist Niels Bohr put forward a theory to overcome the detects in Rutherford’s atomic model. Electrons are moving with high velocity around the nucleus in specified paths called “ orbits or shell ”.As along as the electron is in a particular orbit is its energy is constant.Therefore these orbits are called “stationary orbit or stationary shell ”. Bohr proposed that ‘n’ can take values 1,2,3,4,…etc &named it is the principal quantum number. Each stationary orbit corresponds to a definite energy.There stationary orbit are designated by K,L,M,N,O,… .The orbit close to the nucleus has less energy compared to the orbit away from the nucleus. n=4 n=3 n=2 n=1 + Atomic spectra display fine structure due to splitting of spectral lines. I an attempt to account for the fine structure, Arnold Sommerfeld proposed elliptical orbits instead of circular orbits proposed by Bohr. The atomic spectra I lines,in the presence of applied magnetic field, spit into several lines.This phenomenon is called the ‘Zeeman Effect’. In 1925 Uhlenbeck and Goudsmit suggested that a moving electrons in an orbit revolves or spins on its own axis.this is similar to the earth revolving on its own axis & also revolving around the sun.. Such a spin would contribute to the angular momentum of the electron.The electron can spin clockwise or anti clockwise and this is represent as and respectively.