1. The Story of the Atom Have your books open to page 73 Chapter 3

2. Greek Philosopher’s In 430 B.C. Democritus thought that MATTER was composed of tiny PARTICLES. He believed that if you could take a piece of MATTER, cut it in half again and again, eventually you wouldn’t be able to cut any more because you would only have ONE PARTICLE left. Philosophers named these ATOMS Atom means “ CAN NOT be divided ”. #1

3. Greek Philosopher’s (cont) The philosophers’ theories were results of REASONING, DEBATING and DISCUSSION--- not of evidence or proof! Today, scientists will NOT accept a theory that is not SUPPORTED by EXPERIMENTAL evidence! Scientists came to realize that all matter is made up of ELEMENTS. Elements are matter made of ATOMS of only ONE kind. Example: The element gold contains only gold atoms GOLD By themselves or in combination, the ELEMENTS form every object in the WORLD around you! #1

4. #2 John DALTON In the 1800’s John Dalton combined the idea of element with the Greek theory of the atom #2

5. Dalton (cont) pg73 Models of the Atom Dalton pictured an atom as a hard sphere that was the same throughout. #2

6. William CROOKES In 1870, William Crookes did experiments with a glass tube with very little air/gas inside #3

7. William CROOKES He found that when a battery was connected, the glass tube suddenly lit up with a greenish-colored glow. The cross-shaped shadow showed Crookes that SOMETHING was traveling in a straight line from the CATHODE (-) to the ANODE(+) similar to a beam of a FLASHLIGHT #3

8. William CROOKES The rays were called CATHODE rays. His experiment is known as the CRT (Cathode Ray Tube). Unfortunately, Crookes died before his contributions were recognized. He is given credit for discovering PLASMA (the forth state of matter!) NO MODEL #3

9. JJ THOMSON In 1897, J.J. Thomson continued the Crooke’s investigation and placed a magnet beside the tube from Crookes' experiments. What do you think happened? #4

10. The beam BENT in the direction of the magnet~ In this picture the magnetic pole is repelling the beam. What does this say about the charge of the particles in the CRT? Click pic for link JJ THOMSON Models of the Atom #4

11. Thomson knew light cannot be bent by a magnet; therefore, Thomson concluded that the beam must be made up of charged particles of matter. He discovered the ELECTRON or negatively charged particles which are a part of every ATOM Thomson’s experiments provided evidence that particles smaller than ATOMS do exist!! JJ THOMSON Models of the Atom #4

12. Thomson pictured a sphere with positive charge. The negatively charged electrons were spread evenly within the positive charge. JJ THOMSON Models of the Atom The “ _+_” charges neutralized the “_ - _” charges. The overall charge of the atom was NEUTRAL #4

13. Ernest RUTHERFORD Models of the Atom 1 1 In 1911, Ernest Rutherford and his coworkers began an experiment to find out if Thomson's model of the atom was correct #5

14. Rutherford's Experiments Models of the Atom They wanted to see what would happen when they fired fast-moving, positively charged bits of matter, called alpha particles, at a thin film of a metal such as gold. #5

15. Rutherford predicted that most of the speeding alpha particles would pass right through the foil and only have a MINOR bends Thomson’s Model Expected Results Models of the Atom #5 Electron

16. There wasn't enough charge in any one place in Thomson's model to repel the alpha particle strongly. Expected Results Models of the Atom That was a reasonable hypothesis because in Thomson's model, the positive charge is essentially neutralized by nearby electrons. #5 Electron

17. The Model Fails Models of the Atom To his amazement, a few particles BOUNCED STRAIGHT BACK! He said these results were as likely as shooting a 15” shotgun shell at a piece of tissue paper and having it hit and come straight back at you. Click pic for link #5

18. So, Rutherford hypothesized that most of the atom’s MASS and ALL of its positive charge is crammed in to a very small center of the atom called the NUCLEUS PROTON-a positively charged particle present in the nucleus of all atoms. The Model Fails Models of the Atom #5

19. Rutherford said an atom was mostly open space, with a tiny positively (+) charged nucleus in the center and electrons moving through space. A Model with a Nucleus Models of the Atom #5 Nucleus

20. Rutherford’s new model of the atom fits the experimental data. #5

21. In Rutherford’s model the only other particle in the atom, aside from the electrons, was the PROTON. This meant that the mass of an atom should have been about EQUAL to the mass of the protons, but it was not! The mass of most atoms is at least 2X as great as the mass of the protons. Another particle must be in the nucleus to account for the extra mass???? This will be discovered later! #5

22. Niels Bohr In 1913, Bohr was one of RUTHERFORD’S students. Bohr suggested that the ELECTRONS are found only in SPECIFIC ORBITS around the nucleus and have a fixed energy. Kind of like planets orbiting around the sun or rings in the trunk of a tree. #6

23. Niels Bohr’s Model #6 Nucleus

24. ELECTRON CLOUD In the 1920’s, Scientists determined that electrons do NOT orbit the nucleus like the planets, as Bohr suggested. Instead, electron movement is rapid and constantly UNPREDICTABLE within a “CLOUD” like region around the positively charged nucleus. #7 Nucleus Cloud

25. Modern Cloud Model In 1930’s James Chadwick discovered the missing particle in the nucleus called the NEUTRON. It was hard to find because it has “ NO ” charge. #8

26. Modern Cloud Model Now the model has a nucleus containing both PROTONS and NEUTRONS surrounded by a cloud-like region of moving electrons. New research supports this model; however, the details of the smallest scales of matter are still unknown. #8

27. Modern Cloud Model #8

28. It’s Elemental… An ELEMENT is MATTER made of only ONE kind of atom! There are 118 known elements. 94 NATURALLY occurring elements 24 SYNTHETIC elements- MAN-MADE in laboratories The PERIODIC TABLE is a chart that ORGANIZES and displays information about the ELEMENTS

29. Compounds… COMPOUND -Substance whose smallest unit is made up of atoms of more than one element BONDED together H 2 O Compounds are written in FORMULAS which tell you which elements make up a compound as well as how many atoms of each element are present. The subscript number tells you how many ATOMS of the preceding element are in the compound

30. COMPOUNDS No subscript is used when only ONE atom of an element is present Compounds are always made of the SAME elements in the SAME proportion (water is always H 2 0 never H 3 O)

31. Examples: H 2 O = _____ hydrogen atoms, _____ oxygen atoms (water) H 2 O 2 = _____ hydrogen atoms, ______ oxygen atoms (hydrogen peroxide) C 6 H 12 O 6 = _____ carbon atoms, _____ hydrogen atoms, ______ oxygen atoms (glucose) CaCO 3 = _____ calcium atoms, ______carbon atoms, ______ oxygen atoms (limestone) NaCl= _____ sodium atoms, ______chlorine atoms (salt) N 2 O= _____ nitrogen atoms, ______ oxygen atoms (laughing gas )