1. The History of Atomic Structure

2. Democritus  Named the atom from the Greek word “atomos” meaning indivisible.  Wrote that atoms were the smallest unit of matter to keep its identity.

3. John Dalton  England 1780’s  All matter is made up of atoms.  All atoms of the same element are the same.  Atoms combine in specific ratios.  There is nothing smaller than an atom.

4. J. J. Thomson  Discovered the electron while running electricity through a gas.  Discovered that electrons were negative.  “Plum pudding” model of the atom.

5. Plum Pudding Model  Was using a Crooke’s tubeCrooke’s tube  Found that running electricity through a gaseous element produced a stream of particles

6. Experimented with magnets -

7. Experiment #2 +

8. Conclusions?  The stream of particles was …  Negative  Overall, atoms are...  Neutral  Therefore, atoms must also contain charges that are...  Positive

9. Plum Pudding Model  Thomson figured out that atoms contained positive and negative charges but had no idea how they were arranged.  He guessed there was a diffuse positive cloud with negative charges randomly distributed

10. Marie & Pierre Curie  Provided evidence for the existence of protons and neutrons.  Discovered radium and polonium.  Marie Curie was the first female winner of the Nobel prize and the first double winner.

11. Ernest Rutherford  Discovered that the nucleus was positively charged and very tiny.  Found that the atom is mostly empty space.

12. Ernest Rutherford  Rutherford found the positive particles called alpha particles. He shot these particles at a piece of gold foil and found that many particles went straight through the foil and some were deflected backwards. This does not fit the plum pudding model!  There must be a concentrated positive charge that was repelling his rays!

13. Set-up Detection screen Gold foil (0.00006 cm thick) Uranium Positive particles

14. Hypotheses  If Thomson was correct and atoms contained only diffuse positive charge then…  Most of these heavy positive particles should go right through, right?

15. Experiment Detection screen Gold foil Uranium Positive particles

16. What’s going on?  Most are going straight through  But some are being deflected  How much can they be deflected?

17. Continuation

18. HOLY ATOMS!  What conclusions can be drawn here?  There is something inside an atom that is heavy enough to deflect a particle straight back.  That “something” must be positively charged.  It must also be very small.

19. Conclusions?  Heavy, dense, positively charged, small piece of the atom is….  The nucleus!!  Lets watch this animated Lets watch this animated

20. Niels Bohr  Developed the “solar system” model of the atom.  The nucleus in the center of the atom is made of protons (+) and neutrons.  Electrons (-) orbit the nucleus like the planets around the sun.

21. Electron Cloud Model  The current model of the atom.  Electrons do not orbit the nucleus in orderly paths.  Electrons move around the nucleus at near the speed of light in a general area.  Research continues.

22. Electron Cloud Model Nucleus e-e- e-e- e-e- e-e-

23. Summary

24. Proton Don says “If all the elements are on the periodic table, where’s the cheese?”

25. Protons  Found inside the nucleus of the atom.  Have a charge of +1  Have a mass of 1 atomic mass unit (amu)

26. Neutrons  Found inside the nucleus of the atom.  Mass of 1 amu  Neutrons have no charge.

27. Electrons  Found in orbits (energy levels or shells) around the nucleus.  Travel at near the speed of light.  Charge is –1  Mass is 1/1800 th amu.

28. What makes one element different from another?  The number of protons is always the same as the atomic number.  The number of electrons is usually the same as the atomic number.  To find the number of neutrons: take the atomic mass, rounded to the nearest whole number, and subtract the atomic number.

29. Finding the number of neutrons The mass number of this isotope of lithium is 7. Notice that 7 is equal to the total number of protons and neutrons. If you remove the protons (atomic number), the neutrons are left.

30. Isotopes of Hydrogen What do they have in common? What are the differences?