#02. STRUCTURE OF ATOM

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Discovery of the Electron  J.J. Thomson performed an experiment by passing electricity at a high voltage through a discharge tube containing a gas at a very low pressure. A green fluorescence was seen emitting out from the other end of the discharge tube.  This fluorescence is the result of rays emitted from the cathode (negative plate) towards the anode (positive plate) in the discharge tube. Hence, these rays are called cathode rays.  From his experiment, Thomson arrived at the conclusion that cathode rays are nothing but a stream of negatively charged particles. These negatively charged particles are called electrons. Discovery of the Proton  A German scientist, E. Goldstein in 1886, modified the discharge tube and passed electric current through it.  He found that the positively charged rays were emitted from the anode in the discharge tube. These rays were called canal rays.  When an electric field was applied, these rays deflected towards the negatively charged plate. Thus, Goldstein concluded that an atom contains positively charged particles along with electrons.  These positively charged particles were named as protons by a British scientist, Ernest Rutherford.  Canal rays were also called anode rays since they were emitted from the anode (electrode connected to the positive terminal of high voltage source) in the gas discharge experiments using a perforated cathode. Discovery of the Neutron  In 1932, James Chadwick observed that when beryllium was exposed to α-particles, different kinds of particles were emitted.  These particles had about the same mass as protons and carried no electrical charge. Hence, Chadwick named these particles neutrons.  These were present in the nucleus along with protons.  Neutrons are present in the nucleus of all the atoms except hydrogen.  As protons and neutrons are both present in the nucleus, they are together known as nucleons.

Thomson's Model of an Atom  Thomson's model of an atom is popularly known as the plum pudding or Christmas pudding model of an atom. According to the Thomson's plum pudding model, an atom is a positively charged sphere in which the electrons are embedded.  The negative charge of the electrons and the positive charge of the sphere is equal in magnitude. Thus, an atom as a whole is electrically neutral.  But, his model could not explain the results of experiments carried out by other scientists such as Rutherford and Bohr. Limitations of Thomson's Atomic Model  Although Thomson's atomic model explained why an atom is electrically neutral, it could not explain the distribution of electrons in the atom.  If we accept that electrons are embedded in the positive charge, then the opposite electric charges should cancel each other out and the charged sphere would be uncharged.  Thomson's model could not explain why different elements have different chemical properties.Rutherford's Model of an Atom In 1911, Earnest Rutherford, a scientist from New Zealand, overturned Thomson’s atomic model by his gold foil experiment. Rutherford’s Scattering Experiment  Rutherford selected a gold foil as he wanted a very thin layer.  The gold foil used by Rutherford was 0.004 millimetres in thickness. That is, the foil was about 1000 atoms thick.  In his experiment, fast moving α-particles (alpha particles) were made to fall on a thin gold foil.  The α-particles are helium ions with a +2 charge. Their atomic mass is 4 u. Hence, a high velocity beam of α-particles has a lot of energy.  These particles were studied by means of flashes of light they produced on striking a zinc sulphide screen.  The α-particles are much heavier than the sub-atomic particles present in gold atoms.  Hence, he expected the α-particles to pass through the gold foil with little deflection and strike the fluorescent screen. Rutherford postulated that the atom must contain large empty spaces as most of the α-particles passed through it without getting deflected.  Some α-particles were deflected by the foil through small angles, while some were deflected through very large angles. Thus, Rutherford concluded that the positively charged particles in an atom must be concentrated in a very small space.  One out of every 12,000 particles were deflected through 180o showing a full rebound. Thus, Rutherford came to the conclusion that all the positive charges of the atom and most of the mass of the atom is concentrated in a very small volume within the atom.  Rutherford named this small space inside the atom as the nucleus of the atom or the atomic nucleus.  On the basis of these observations, Rutherford calculated that the atomic nucleus is 105 times smaller than the total area of the atom.  The radius of the atom is 10-8 centimetres while the radius of the nucleus is 10-13 centimetres.  Thus, we can say that the atom is relatively hollow with a heavy nucleus at its centre. The electrons arranged around the nucleus possess negligible mass.  Based on his observations, he formulated the Theory of atom.

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