1. J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these negative particles. He knew that atoms did not have a net negative charge and so there must be balancing the negative charge. J.J. Thomson

2. William Thomson (Lord Kelvin) In 1910 proposed the Plum Pudding model –Negative electrons were embedded into a positively charged spherical cloud. Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 56 Spherical cloud of Positive charge Electrons

3. Ernest Rutherford (1871-1937) Learned physics in J.J. Thomson’ lab. Noticed that ‘alpha’ particles were sometime deflected by something in the air. Gold-foil experiment Rutherford PAPER Rutherford PAPER Animation by Raymond Chang Animation by Raymond Chang – All rights reserved.

4. Rutherford ‘Scattering’ In 1909 Rutherford undertook a series of experiments He fired  (alpha) particles at a very thin sample of gold foil According to the Thomson model the  particles would only be slightly deflected Rutherford discovered that they were deflected through large angles and could even be reflected straight back to the source particle source Lead collimator Gold foil  

5. Rutherford’s Apparatus beam of alpha particles radioactive substance gold foil circular ZnS - coated fluorescent screen Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 120 Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemistry.1908 Nobel Prize in Chemistry

6. What He Expected The alpha particles to pass through without changing direction (very much) Because The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles California WEB

7. Interpreting the Observed Deflections Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 120.............. gold foil deflected particle undeflected particles.. beam of alpha particles.

8. Density and the Atom Since most of the particles went through, the atom was mostly empty. Because the alpha rays were deflected so much, the positive pieces it was striking were heavy. Small volume and big mass This small positive area is the nucleus California WEB

9. Table: hypothetical description of alpha particles alpha rays don’t diffract alpha rays deflect towards a negatively charged plate and away from a positively charged plate alpha rays are deflected only slightly by an electric field; a cathode ray passing through the same field is deflected strongly... alpha radiation is a stream of particles... alpha particles have a positive charge... alpha particles either have much lower charge or much greater mass than electrons observation hypothesis (based on properties of alpha radiation) Copyright © 1997-2005 by Fred SeneseFred Senese

10. Explanation of Alpha-Scattering Results Plum-pudding atom + + + + + + + + - - - - - - - - Alpha particles Nuclear atom Nucleus Thomson’s modelRutherford’s model

11. Results of foil experiment if plum- pudding had been correct. Electrons scattered throughout positive charges Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 57 + + + + + + + + - - - - - - - -

12. This is the modern atom model. Electrons are in constant motion around the nucleus, protons and neutrons jiggle within the nucleus, and quarks jiggle within the protons and neutrons. This picture is quite distorted. If we drew the atom to scale and made protons and neutrons a centimeter in diameter, then the electrons and quarks would be less than the diameter of a hair and the entire atom's diameter would be greater than the length of thirty football fields! 99.999999999999% of an atom's volume is just empty space! Website “The Particle Adventure”The Particle Adventure