1. Atomic Theory

2. Catalyst (5 min) What does the atomic number of each atom represent?
What does the mass number of each atom represent?
How many protons and neutrons are in the nuclei of the following elements?
phosphorus (P)
cadmium (Cd)
lead (Pb)

3. Let’s Take a Trip Through Time!

4. ``Nothing exists except atoms and empty space; everything else is opinion.'' 
Democritus

5. Democritus 460 – 370 B.C.
the first to propose that matter was not infinitely divisible
Believed that matter was made up of tiny indivisible particles called atomos
Everything is composed of small atoms moving in a void
Ideas rejected by leading philosophers because they did not believe in empty space

6. First Concept of an Atom

7. John Dalton 1766-1844 English School Teacher
Introduced his ideas in 1803
Used experiments that studied the ratio that elements combine in chemical reactions to transform Democritus’s ideas on atoms into a scientific theory
Became the head of his quaker school at 12, was also reading newton’s principia at that time. French chemist pelletier traveled to meet the famous scientist and was shocked to see him teaching arithmetic to elementary boys.

8. Dalton’s Atomic Theory
All elements are composed of tiny indivisible particles called atoms
Atoms of the same element are identical. The atoms of any one element are different from those of any other element.
Atoms of different elements can physically mix together or chemically combine to form compounds
In a chemical reaction, atoms are separated, combined or rerranged. Atoms of one element are never changed into atoms of another element.
Atoms cannot be created, divided into smaller particles or destroyed.

9. Dalton’s Model

10. But if the scientists couldn’t SEE the atom, how did they even know it existed?!

11. ``To the electron: may it never be of any use!''
J.J. Thomson

12. J.J. Thomson
Hypothesized that the structure of an atom was like ‘plum pudding’
Discovered the electron in the Cathode Ray Experiment
Discovered that this subatomic particle has a negative charge
Discovered isotopes 1913

13. Cathode Ray Tube Experiment
Identified the first subatomic particle, the electron
Determined the charge to mass ratio of the electron
Mass of the electron is MUCH less then the mass of a hydrogen atom
Meant there are particles smaller than atoms!

14. Video!

15. Cathode Ray Experiment
Altering the gas in the tube and the material used for the cathode have no effect
Proves: the ray’s negative particles (electrons) are found in all forms of matter
Cathode Ray is deflected toward the positively charged plate by an electric field
Proves: the particles in the ray must have a negative charge

16. Plum Pudding Model

17. “Fullness of knowledge always means some understanding of the depths of our ignorance; and that is always conducive to humility and reverence. “
Robert Millikan

18. Robert A. Millikan
Conduced oil drop experiment to determine the mass and quantity of an electrons charge

19. “You should never bet against anything in science at odds of more than about 1012 to 1.”
Ernest Rutherford

20. Ernest Rutherford 1871-1937 Student of J.J. Thomson
Conducted the gold foil experiment
Discovered that an atom consists of mostly empty space through which the electrons move
Concluded there is a tiny dense region called the nucleus which is located in the center of the atoms and contain all its mass
Electrons are held in the atom by attraction with positively charged protons.
Wasn’t particularly great at mathematics, in lectures would often get lost in his own equations and ask his students to finish. According to colleague James chadwick, also wasn’t particularly clever at experimentation. He was simply tenacious and open minded. He was prepared to work longer and harder then most. His greatest discovery came when he spent tedious hours counting alpha particles – work that would normally be farmed out.

21. Gold Foil Experiment Shot alpha particles at a thin piece of gold foil
Most of particles passed through foil which indicated that atoms are composed of mostly empty space
Alpha particle sometimes bounced back which indicated they were deflected from the nucleus

22. Video!

23. “If an atom were expanded to the size of a cathedral, the nucleus would be about the size of a fly - but a fly many thousands of times heavier than the cathedral.”

24. “This is almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you”
Ernest Rutherford

25. Rutherford Model

26. But what keeps the electrons from spiraling into the nucleus?

27. ``An expert is a person who has made all the mistakes that can be made in a very narrow field.''
Enter niels bohr
Niels Bohr

28. Niels Bohr
Developed theory that electrons orbited the nucleus in neat, planet-like orbits
Planetary Model 1913
Nucleus surrounded by orbiting electrons at different energy levels
Electrons have definite orbits
Utilized Planck’s Quantum Energy theory
Worked on the Manhattan Project (US atomic bomb)
Bohr was a student of Rutherford’s, who postponed his honeymoon when he first came up with the idea that electrons had specific orbits. One the nobel prize for his work the year after einstein

29. Bohr Model for Nitrogen

30. How can the protons bundle inside the nucleus without blowing the atom apart?

31. James Chadwick 1932
showed that the nucleus contained a neutral particle called the neutron.
Enter James chadwick, a colleague of rutherfords at cambridge who spent 11 years of his life to finding the neutron

32. Quantum Model (learn next class)
Developed from wave equation by Erwin Schrodinger (1926)

33. Fill in the chart using your notes (5 min)
Name
Democritus
Dalton
Thomson
Rutherford
Bohr
Time Frame
Key Points
Model

34. Who Did It? (5 min)
Six scientists were sitting around a table discussing their contributions to Atomic Theory. Use the clues to determine where they were sitting and what contribution they made.

35. Subatomic Particles Table Summary
In your notes fill in the table comparing the relative charge, mass and location of each of the subatomic particles (3 min)

36. In your notes, make a timeline graph showing the development of modern atomic theory (5 min)

37. You Do:
Complete the following book problems on a separate sheet of paper: p.104 #1, 4 p.105 #13, 15 p.122 #39, 62

38. Closing
How did the concept of the atom change over time and what key experiments and scientists led to our current understanding of the atom?

39. Homework
Describe the location of the electrons in Thompson’s ‘plum pudding’ model of the atom
In the Rutherford atomic model, which subatomic particles are located in the nucleus?
Describe Thomson’s and Millikan’s contributions to atomic theory.