Structure Of Atom - Part 1 - PDF

Structure Of Atom  Gases are bad conductors of electricity. However under reduced pressure and at high potential different gases conduct electric current. DISCHARGE TUBE Discharge tube experiment At low pressure and at very high potential difference, electric current is passed throu...

Structure Of Atom  Gases are bad conductors of electricity. However under reduced pressure and at high potential different gases conduct electric current. DISCHARGE TUBE Discharge tube experiment At low pressure and at very high potential difference, electric current is passed through a gas. Observations At 1cm Hg pressure and a potential difference of a few thousand volts , a a flash of light is seen. At 1 mm Hg , the tube is mostly filled with a glow extending from the positive electrode At 0.001 mm Hg, the glow disappears and the walls of the glass tube begin to glow with a brilliant green light. Conclusion Some radiation or rays emanated from cathode. These rays are known as "Cathode Rays" Using different electrodes and different gases the same value for charge to mass ratio for the particles that make up cathode rays was obtained. Cathode rays consist of particles known as electrons. PROPERTIES OF CATHODE RAYS They travel in straight lines. They are independent of the material composition of the cathode. Cathode rays deflect towards positively charged plate in an electric field.Hence cathode ray consists of negatively charged particles. Cathode rays penetrate small thickness of matter such as aluminium foil, gold foil. Discovery of electron - 1887 J.J. Thomson (1856-1940) Towards the end of the 19th century Joseph J.Thomson was studying electric discharges at the well-known Cavendish laboratory in Cambridge, England to study the properties of the rays coming from the cathode (Kennedy, Chemistry Live!)  By carefully measuring deflection of cathode rays by electric and magnetic fields, Thomson determined the ratio between the electric charge (e) and the mass (m) of the rays. e/m = -1.76 x 10-11 coulombs/kg. Millikan oil-drop experiment - determination of electron charge (1909) Robert Millikan (1868 – 1953) Millikan and Harvey Fletcher measured electric force and electric field on an isolated charged oil droplet. Millikan found that the charge on individual oil drops was always whole-number multiples of 1.602 10−19. This is the elementary electric charge (1.602 10−19 coulomb) Mass of an electron Thomson’s model of an atom( plum pudding model) Electrons are embedded in a sphere of positive charge. Discovery of nucleus - 1911 Ernest Rutherford (1871-1937) was awarded Nobel prize in 1908 for his work in radioactivity. Observations Almost all the alpha particles passed through the foil. Some alpha particles were deflected off at different angles Very few of the alpha particles (one or two) bounced backwards after hitting the gold foil. Conclusion- Rutherford’s model of atom Most of the space within the atoms is empty. The alpha particles which were deflected by large angles must have approached some positively charged central region (nucleus) The volume occupied by nucleus is very small. Almost complete mass of the atom must lie in the nucleus. Drawbacks of Rutherford’s model Not in accordance with Maxwell’s theory and could not explain the stability of an atom. Did not say anything about the arrangement of electrons in an atom which made his theory incomplete. Discovery of Protons In 1886 Goldstein discovered the existence of positively charged rays in the discharge tube by using perforated cathode. These rays were named as anode rays or cannal rays. In 1920 ,Rutherford proposed the name proton for the positively charged particle in the nucleus of an atom. Discovery of Neutron Various experiments showed that mass of the nucleus is approximately twice than the number of proton. Rutherford postulated the existence of some neutral particle having mass similar to proton. In 1932, James Chadwick bombarded Beryllium metal with alpha particles resulting in the emission of highly energetic radiation By analyzing the energies of different targets after bombardment he discovered the existence of a new particle which is charge less and has similar mass to proton. This particle was named neutron. A student of Rutherford , James Chadwick was awarded Nobel Prize in physics in 1935 for discovering neutron Quantum Theory and the Atom Niels Bohr (1885 – 1962)