The Particle Nature of Atom

1. The Particle Nature of Atom Chapter 2: Particle-like behavior of radiation Chapter 4: Matter Waves What is matter composed of? Chapter 3: Faraday (1833): The law of electrolysis: The mass of an element librated at an electrode is directly proportional to the charge transferred and to the atomic weight of the librated material. Thompson (1897): The identification of cathode rays as electrons and the measurement of the (e/m) of electrons. Millikan (1909): Precise measurement of the charge of electron. Rutherford (1913): The establishment of the Nuclear model of the atom.

2. Faraday’s Experiment Electrolysis of molten common salt (NaCl) If 96 500 C of charge (1 faraday) is passed through the solution, 23.0 g of Na will deposit on the cathode and 35.5 g of Cl gas will bubble off the anode. In this case only one gram atomic weight or mole of each element is released because both are monovalent. Doubling the quantity of charge passed double the mass librated. Faraday’s results is given in the equation M= (q)(molar mass)/(96500 C)(valence) Under the influence of the electric field produced by the battery, ions move to the anode or the cathode where they lose or gain electrons and are librated as neutral atoms. Confirmed three parts of atomic picture • Proof that matter consist of molecules and molecules consist of atoms • Charge is quantized 3. Subatomic parts of the atom are negative and positive

3. Thompson’s (e/m) Experiment For 30 years: Were cathode rays material particles or waves? The diagram: Electrons are accelerated from the cathode to the anode. Electric field alone will deflect the beam in the y-direction. Applying a magnetic field will make the beam undeflected. From the size of the deflection and the measured values of E and B, the charge-to-mass ratio can be determined. The measurement of the horizontal velocity component of the beam by balancing the forces from the electric field and the magnetic field.

4. Millikan’s Oil-Drop Experiment (1899) By observing Single charged droplets he was able to measure the electronic charge Oil droplets charged by an atomizer are allowed to pass through a small hole in the upper plate of a parallel-plate capacitor. If the droplets are illuminated by the side they appear as brilliant stars and the rate of fall of individual droplet can be measured. In an electrostatic field of several thousands volts is applied to the plates of the capacitor, the drop will move slowly, so the rate could be followed It was observed a discontinuous change or jump to a different velocity. This is caused by attraction of ions to the drop changing the charge on the drop Such changes become more frequent when a source of ionization radiation is placed

5. Rutherford Model of the Atom Noticed that a beam of finely collimated alpha particles was broadened when passed through a metal foil yet easily penetrated the foil. Studies the distribution of mass by observing the scattering of the alpha particles. Rutherford discovered that most of the atomic mass and the positive charge lie in a minute central nucleus of the atom Experiment: Alpha particles of speed 2x107 m/s struck a gold foil several thousand atomic layers thick. Most of the alpha particles passed. But some scattered at angle phi. They were counted. Head-on collision with the nucleus would only deflect the alpha particles a small angle. If however, the positive charge is concentrated at the center of the atom, the electric repulsion will cause large deflection