History of Atomic Theory Chapter 5 - Part 2 Objectives - 1 List the 4 parts of Daltons Atomic Theory ad explain its significance to the history of modern atomic theory. Diagram and explain the significance of a Crookes Tube. Explain how the beam must have been negatively charged particles. Explain how Thomson discovered the existence of the proton, the electron, and found the charge-to- mass ratio of the electron. Explain and draw a picture of the Plum Pudding Model. Objectives - 2 Explain and draw a picture of Millikans Oil Drop Experiment. Explain how the Oil Drop Exp. lead to the discovery of the charge on the electron. Explain how a fundamental principle of the ODE was like charges repel. State the discovery of William Roentgen. State the discovery of Henri Becquerel. Democritus of Abdera Ancient Greece - 4th century BC first suggested the existence of tiny fundamental particles that make up matter. atoms = indestructible did not agree with the current sci theory - but did not explain chemical behavior - and lacked experimental support. John Dalton Eaglesfield, England Father of modern atomic theory Pro: advance in the description and nature of chemical reactions. Con: did not hold up when subatomic particles were disovered Daltons Atomic Theory 1. All elements are composed of tiny individual particles - called atoms. 2. All matter is composed of combinations of these atoms. 3. Atoms of different elements are different. 4. Atoms of the same element have the same size, mass, and form. Daltons Atom The atom is a solid indestructible sphere. William Crookes British physicist investigated why gases glow Crookes Tube Crookes Experiment used a partially evacuated tube with a low pressure gas and ran a current through it power on: a green glow was emitted as a beam of light ran across the tube If the charge on the plates as switched... Crookes Outcomes It was a beam of particles? How did he know? Determined the particles had a negative charge - due to... Created a cathode ray tube (also called a Crookes Tube) This was the precursor to the TV tube - Thank you Crookes!!! JJ Thomson British physicist continued Crookes work Experiment: applied an electric field which bent the light inside the tube back to a straight path and was able to measure this field. Thomsons Outcomes discovered the electron used the strength of the field was able to determine the charge-to-mass ratio of this negative particle beam - now so named determined the existence of the proton (because of neutrality) Thomsons Model Plum Pudding Model Negatively charged electrons embedded in a positive mass of protonness JJs Pros & Cons Pro: did explain some of the electrical nature of matter. Con: did not say anything about the number of charged particles or their arrangement. Won NOBEL PRIZE in 1906 Robert Millikan Millikan & Einstein Robert Millikan US Physicist NOBEL PRIZE 1932 Oil Drop Experiment Millikans Outcomes determined the charge on a single electron using the charge-to-mass ratio from Thomson, many scientists determined the mass of an electron Wilhelm Roentgen William Roentgen Discovered x-rays killed his wife Henri Becquerel French scientist Left a sample of uranium ore in a drawer with glass photo plates and there was fogging on the plates serendipity Becquerels outcomes concluded something in the sample itself must be capable of fogging the plates; the sample must give off some kind of ray without the influence of the sun rays were the product of radioactivity credited with the discovery of radioactivity Marie & Pierre Curie Pierre, Pierres Assistant, and Marie Curie Pierre & Marie Curie The Curie Family: Pierre, Marie, Irene, and Grandfather Curie Marie & Pierre Curie French physicists intrigued with Becquerels work studied radioactivity Marie discovered POLONIUM Ernest Rutherford New Zealand discovered the nucleus of the atom Planetary Motion Model Gold Foil Experiment Gold Foil Experiment - 2 A beam of alpha particles was directed at a thin sheet of gold foil with a fluorescent screen around it. Most of the alpha particles went straight through it - or had very little deflection. A few were deflected back at very sharp angles. A 15-inch shell fired at a piece of tissue paper and it came back and hit you. Rutherford concludes... Atoms have almost all of their mass concentrated in a very small positively charged region = the nucleus The nucleus is surrounded by electrons which are at a relatively large distance. Why did the electrons not crash into the nucleus in Rutherfords model? Planetary Motion Model Electrons orbit around the nucleus like planets orbit around the sun. The nucleus held (only) protons (for the time being). Hans Geiger Student of Rutherford He was one of the people who had to spend long hours (in mostly darkness) counting the particles that hit the screen. Created the first electrical particle counter - The Geiger counter Geigers contributions Determined that alpha particles were made up of 2 protons and 2 neutrons. (1908) This gives an alpha particle a mass number of ??? and a nuclear charge of ???. Geiger - Rutherfords student Curie-Joliot & Joliot Irene Curie-Joliot & Frederic Joliot Bombarded beryllium with alpha particles - which created a beam of energy that had great power. It was able to penetrate metals. They thought it was a new form of energy; they thought it was gamma rays. James Chadwick British physicist and chemist Found the Curie-Joliot beam of energy was unaffected by magnetic fields - meaning it must be neutral. Particles moved at 1/10 the speed of light - so it was NOT radiant energy - it must be PARTICLES Chadwicks Particles The new particles had approximately the same mass as a proton - but these new particles had NO CHARGE. He is credited with the discovery of the neutron. after Chadwick After neutrons were discovered, isotopes of various elements were discovered. Neils Bohr Bohr & Einstein Bohr continued with energy calculations and he further developed the planetary motion model to include neutrons, and specific energies of electrons in a certain path. By no means have we finished our story. We come revisit atomic history in a little bit to learn more about what other theories have developed and find out where we are now.