Title 2.STRUCTURE OF ATOM.pdf ✅

ChemContent | Structure of Atom ChemContent 1 STRUCTURE OF ATOM Introduction: The word "atom" has been derived from the Greek word 'atoms’ which mans 'indivisible’. These early ideas were mere speculation and there was no way to test them experimentally. Atomic Structure: Atom is made up of smaller units like proton, neutron and electron. Some other particles like positron, neutrino, antineutrino, π-meson, μ-meson, k meson etc are also present which are very short lived. Discovery of Electron In 1879, William Crooks studied the conduction of electricity through gases at low pressure. He performed the experiment in a discharge tube which is a cylindrical hard glass tube about 60 cm in length. It is sealed at both the ends and fitted with two metal electrodes. The electrical discharge through the gases could be observed only at very low pressures and at very high voltages. J.J. Thomson took a discharge tube and applied a voltage of a 10000 volt potential difference across it at a pressure of 10–2 mm of Hg. He found some glowing behind anode. It means some invisible rays produced at cathode strike behind anode and produce fluorescence. He named them cathode rays. Properties of Cathode Rays i. These rays have mechanical energy and travel in straight line.
ChemContent | Structure of Atom ChemContent 2 ii. These rays are deflected towards positive plate of electric field. It means these are made up of negatively charged particle called electron. iii. Colour observed is independent from nature of gas. iv. Mulliken determined the charge on electron which is 1.602 × 10-19C. v. Specific charge on electron is calculated by J.J. Thomson. Charge to mass ratio J.J. Thomson for the first time experimentally determined charge/mass ratio called e/m ratio for the electrons. For this, he subjected the beam of electrons released in the discharge tube as cathode rays to influence the electric and magnetic fields. These were acting perpendicular to one another as well as to the path followed by electrons. According to Thomson, the amount of deviation of the particles from their path in presence of electrical and magnetic field depends on, 1. Magnitude of the negative charge on particle 2. Mass of particle 3. Strength of magnetic field When electric field is applied, deviation from path takes place. If only electric field is applied, cathode rays strike at A. If only magnetic field is applied, cathode rays strike at C. In absence of any field, cathode rays strike at B. By carrying out accurate measurements on the amount of deflections observed by the electrons on the electric field strength or magnetic field strength, Thomson was able to determine the value of e/me = 1.758820 x 1011 C kg-1 where me = Mass of the electron in kg e = magnitude of charge on the electron in coulomb (C). Discovery of anode rays In 1886, Goldstein modified the discharge tube by using a perforated cathode. On reducing the pressure, he observed a new type of luminous rays passing through the holes or perforations of the cathode and moving in a direction opposite to the cathode rays. These rays were named as positive rays or anode rays or as canal rays. Anode rays are not emitted from the anode but from a space between anode and cathode. Properties of anode rays
ChemContent | Structure of Atom ChemContent 3 1. These rays deflect towards negative plate of applied electric field. It means these are made up of positively charged particle. 2. Property of anode rays depends on nature of gas. 3. These rays travel in straight line and have mechanical energy. Discovery of Neutron Chadwick in 1932 found the evidence for the production of neutron in given reaction. 4Be9 + 2He4 ⟶ 6C 12 + 0n 1 Neutron is chargeless particle and have mass equal to proton. Millikan’s Oil Drop Experiment In this experiment, some fine oil droplets were allowed to enter through a tiny hole into the upper plate of electrical condenser. These oil droplets were produced by atomiser. The air in the chamber was subjected to the ionization by X-rays. The electrons produced by the ionization of air attach themselves to the oil drops. Thus oil droplets acquire negative charge. When sufficient amount of electric field is applied, the motion of the droplets can be accelerated, retarded or made stationary. Millikan observed that the smallest charge found on them was –1.6 × 10–19 coulomb and the magnitude of electrical charge, q on the droplets is always an integral multiple of the electrical charge ‘e’ i.e., q = ne Thomson’s Model of Atom J.J. Thomson in 1898, proposed a model of atom which looked more or less like plum pudding or raisin pudding. He assumed atom to be a spherical body in which electrons are unevenly distributed in a sphere having positive charge which balance the electron’s charge. It is called Plum pudding model.
ChemContent | Structure of Atom ChemContent 4 Important Feature of This Model: The mass of the atom is assumed to be uniformly distributed over whole atom. Failure: This model was able to explain the overall neutrality of the atom, it could not satisfactorily, explain the results of scattering experiments carried out by Rutherford in 1911. Rutherford's Model Rutherford in 1911, performed some scattering experiments in which he bombarded thin foils of metals like gold, silver, platinum or copper with a beam of fast moving a-particles. The thin gold foil had a circular fluorescent zinc sulphide screen around it. Whenever a-particles struck the screen, a tiny flash of light was produced at that point. From these experiments, he made the following observations: 1. Most of the α-particles pass without any deviation. 2. Few particles deviate with small angle. 3. Rare particles retrace its path or show deflection greater than 90°. On the basis of these observation, he proposed a model. 1. Atom is of spherical shape having size of order 10–10 meters. 2. Whole mass is concentrated in centre called nucleus having size of order 10–15 meters. 3. Electron revolves around the nucleus in circular path like planets revolve around sun. Limitation: This model could not explain stability of atom. According to Maxwell's classic theory, an accelerated charged particle liberates energy. So, during revolution, it must radiate energy and by following the spiral path it should comes on nucleus.