1. 1 Atomic Theory The Atom Copyright © 2008 b Pearson Education, Inc. Publishing as Benjamin Cummings

2. 2 Dalton’s Atomic Theory First to propose “theories” on the atom in 1803 *There were 4 postulates *Problems with the first two *He never ran experiments or provided evidence, and therefore; they were not widely accepted by the science community *biggest issue was that JD thought the atom was the smallest particle. Little did he know that there was much more

3. 3 Passed an electric current through gases at low pressure and sealed the gas fitted with electrodes The resulting glowing beam or cathode ray traveled from the cathode to the anode and was deflected by a magnet; therefore had a (-) charge. Thomson’s Cathode Ray experiment and plum pudding model

4. Thompson The positive side of the magnet attracted the ray and the negative side repelled it and therefore it was determined that there were particles inside the atom that were negative He determined the charge of an electron He called them electrons 4

5. Millikan’s Oil Drop Experiment (1909) Determine the mass to charge ratio of an electron and used Thompson’s findings to calculate the mass of an electron Millikan Oil Drop Experiment 5

6. 6 Rutherford’s Gold-Foil Experiment In Rutherford’s gold-foil experiment Positively charged particles were aimed at atoms of gold. Most went straight through the atoms. Only a few were deflected.

7. 7 Rutherford’s Gold-Foil Experiment (1910) Rutherford Goild Foil Experiment

8. Rutherford’s Gold Foil Conclusions There must be a small, dense, positively charged nucleus in the atom that deflects positive particles that come close. The atom is mostly empty space 8

9. James Chadwick Worked with Rutherford who was convinced there was another particle. After all the atomic number and atomic mass are not the same which indicates there must have been an additional particle 1932-Chadwick discovered by bombarding beryllium with alpha particles. The particle released was the same mass but no charge. 9

10. 10 Subatomic Particles Atoms contains subatomic particles Protons have a positive (+) charge. Electrons have a negative (-) charge. Like charges repel and unlike charges attract. Neutrons are neutral.

11. 11 Structure of the Atom An atom consists Of a nucleus that contains protons and neutrons. Of electrons in the large empty space around the nucleus.

12. 12 Atomic Mass Scale On the atomic mass scale for subatomic particles 1 atomic mass unit (amu) is equal to 1/12 of the mass of the carbon-12 atom. A proton has a mass of about 1 (1.007) amu. A neutron has a mass of about 1 (1.008) amu. An electron has a very small mass (0.00055 amu).

13. 13 Particles in the Atom Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings Table 4.5