Democritus of Abdera. John Dalton. Dalton s Atom. Dalton s Atomic Theory Ancient Greece - 4th century BC. Eaglesfield, England

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1 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 Dalton s Atomic Theory Dalton s Atom 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. The atom is a solid indestructible sphere. 4.Atoms of the same element have the same size, mass, and form.

2 William Crookes Crookes Tube British physicist investigated why gases glow Crookes Experiment Crookes Outcomes It was a beam of particles? 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... 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!!!

3 JJ Thomson Thomson s Outcomes 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. 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) Thomson s Model JJ s Pros & Cons Plum Pudding Model Negatively charged electrons embedded in a positive mass of proton ness 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

4 Robert Millikan Oil Drop Experiment US Physicist NOBEL PRIZE 1932 Millikan s Outcomes William Roentgen determined the charge on a single electron using the charge-to-mass ratio from Thomson, many scientists determined the mass of an electron Discovered x-rays killed his wife

5 Henri Becquerel Becquerel s outcomes French scientist Left a sample of uranium ore in a drawer with glass photo plates and there was fogging on the plates serendipity 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 French physicists intrigued with Becquerel s work studied radioactivity Marie discovered POLONIUM Ernest Rutherford New Zealand discovered the nucleus of the atom Planetary Motion Model

6 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... Planetary Motion Model 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 Rutherford s model? Electrons orbit around the nucleus like planets orbit around the sun. The nucleus held (only) protons (for the time being).

7 Hans Geiger Created the first electrical particle counter - The Geiger counter 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. Geiger s contributions Irene Curie-Joliot & Frederic Joliot 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???. 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.

8 James Chadwick Chadwick s Particles 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 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 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.