Chapter 2 The Atom

History of the Atom Greek philosophers first proposed that matter was composed of minute particles

John Dalton John Dalton(1800)- All matter composed of small particles called atoms Atoms are indivisible cannot be broken down into smaller particles. Atoms cannot be created or destroyed

W Crooks W Crooks: Vacuum tube-Showed the existence of radiation. Simple cathode ray tube

J.J. Thompson J.J.Thompson(1895): By using the cathode ray tube, Thompson showed that the rays consisted of negatively charged particles called electrons. Able to calculate the ratio of charge to mass (e/m). He proposed a simple structure to the atom. Visualised the atom as a sphere with positive charges and electrons embedded in the sphere at random.(plum pudding) http://www.aip.org/history/electron/jjappara.htm

Robert Millikan Calculated the charge and mass of the electron. Oil drop experiment

Oil Drop Experiment oil droplets . . . . . . . . . . . . Charged plate Robert Millikan Charged plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . oil atomizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . + Telescope Small hole Millikan oil drop experiment. A fine mist of oil droplets is introduced into the chamber. The gas molecules inside the chamber are ionized (split into electrons and positive ions) by a beam of x-rays (not represented). The electrons adhere to the oil droplets, some droplets having one electron, some two electrons, and so forth. These negatively charged oil droplets fall under the force of gravity into the region between the electrically charged plates. If you carefully adjust the voltage on the plates, the force of gravity can be exactly counterbalanced by the attractive force between the negative oil drop and upper, positively charged plate. Analysis of these forces leads to a value for the charge on thee electron. Robert Andrews Millikan (1868 – 1953) won the Nobel physics prize in 1923 for his work in isolating and weighing the electron. oil droplet under observation - Charged plate

Earnest Rutherford(1911): Gold leaf experiment-alpha particles Discovery of the nucleus. Discovered protons- alpha particles ejected positive particles from Oxygen and Nitrogen atoms.

James Chadwick(1932): James Chadwick(1932): bombarded samples of beryllium with alpha particles. Discovered radiation consisting of particles with no charge called neutrons.

Properties of Sub-Atomic Particles