1. Atomic Theory

2. Let’s Take a Trip Through Time!

3. Democritus vs. Aristotle
400BC BC

4. Democritus 460 – 370 B.C. Matter composed of Atoms
basic elements from which all matter is made
atoms move in a void
(empty space)
atoms have different shapes (round, pointy, oily, have hooks, etc.)
Ideas rejected by leading philosophers because void = no existence

5. First Concept of an Atom
Atoms are solid, homogeneous, indivisible particles

6. Aristotle 350 BC
Matter can be subdivided into smaller and smaller particles forever
Matter composed of combination of 4 elements:
air, earth, water and fire
No such thing as empty space – had the “ether”

7. Aristotle won that debate!
For 2000 years most people believed that the 4 elements were:
air, earth, water, and fire There was no such thing as empty space!

8. Phlogiston Theory Around 1700’s:
wood  calx (ash) + phlogiston (↑ air)
iron  calx (rust) + phlogiston (↑ air)

9. Antoine LaVoisier Demonstrated importance of measurements in chemistry
Disproved the phlogiston theory
First to state Law of Conservation of Mass

10. LaVoisier was the father of modern chemistry, but he lost his head in 1794!

11. J. Proust – 1797 Law of Definite Proportions
In pure compound: elements combine in definite proportions to each other
H2O
CO2

12. John Dalton

13. Dalton’s Model
Billard Ball model

14. Dalton’s Atomic Theory
Matter is composed of very small particles called atoms
All atoms of a given element are identical
Atoms of different elements are different
Atoms are indivisible and cannot be created or destroyed
Different atoms combine in whole number ratios to form compounds
In a reaction, atoms are rearranged

15. So Dalton’s atoms are kind of like billiard balls
Atoms combine in whole-number ratios

16. In a reaction, atoms are rearranged+ +  +

17. 1904 Thomson’s Model Plum Pudding model (aka:
chocolate chip cookie model)

18. J.J. Thomson 1856-1940 Discovered electron 1897
Cathode Ray Experiment
Discovered isotopes 1913

19. Cathode Ray Tube Experiment
Thought cathode rays were streams of particles smaller than atoms
magnetic field will deflect beam of charged particles

20. Thomson’s discovery meant that the atom WAS divisible!

21. JJ Thomson’s Ideas
matter is neutral!

22. Plum Pudding Model
Negative electrons in a soup of positive charges

23. positive charge evenly spread out while the negative charge is in bits – like chocolate chips in cookies
source

24. Proton – Discovered by 1920 Thomson & Goldstein – 1907
- discovered heavy particle with positive charge
Rutherford – 1918
- shot alpha particles at nitrogen gas and got hydrogen
- hydrogen had to come from nitrogen
- suggested hydrogen nucleus was a
particle & named it the proton

25. Rutherford’s Model
1910
Nuclear model

26. Ernest Rutherford
atom’s mass is mostly in the center of atom
Nucleus
nucleus has a positive charge

27. Rutherford’s Experiment - 1911

28. Compared to electron, alpha particle is massive & fast
Rutherford’s exp’t: animation
Compared to electron, alpha particle is massive & fast
If Thomson’s model correct, alpha particle wouldn’t be affected
- electrons are tiny
- positive charge uniformly spread

29. What did Rutherford conclude from the particles that bounced back?
What did Rutherford conclude from the particles that went straight through?
mostly empty space
What did Rutherford conclude from the particles that bounced back?
hit something really heavy &
positively charged
source

30. Results of Rutherford’s Expt
Results DO NOT match Thomson’s model

31. Rutherford Model
NUCLEAR Model

32. Rutherford proposed nuclear atom : dense central core (+ charged) surrounded by mostly empty space where electrons are found
Rutherford did not speculate on how the electrons were arranged around the nucleus

33. So how big is the nucleus compared to the entire atom?
If atom as big as football stadium
nucleus smaller than flea on 50-yard line!
If atom as big as period at end of sentence in textbook
it would have mass of 70 cars!

34. So how big is an atom? Most atoms are 1-2 angstroms across
An angstrom = 1 X m
6 BILLION Cu atoms in a line = less than 1 meter!
Can we “see” an individual atom?

35. James Chadwick 1891-1974 Worked with Rutherford
Discovered Neutron 1932
Nobel Prize in Physics 1935

36. Bohr’s Model
1913
Planetary model

37. Niels Bohr
His Model:
Nucleus surrounded by orbiting electrons at different energy levels
Electrons have definite orbits
Worked on the Manhattan Project (US atomic bomb)

38. Bohr’s Model Electrons travel only in specific orbits
Each orbit has definite energy
Inner orbit: least energy
Outer orbit: most energy
Atoms emit radiation when e- jumps from outer orbit to inner orbit
Outermost orbits determine atom’s chemical properties

39. Bohr Model for Nitrogen

40. Quantum Mechanical model
Modern Model
1926
Quantum Mechanical model

41. Ernst Schrödinger 1887-1961 Quantum Mechanical Model
Electron’s energy is quantized (has only certain values)
Electrons in probability zones called “orbitals”, not orbits
- location cannot be pinpointed
Electrons are particles & waves at same time
Electrons move around nucleus at speed of light

42. Orbitals

44. The development of atomic theory represents the work of many scientists over many years

45. Which one of you will develop a better theory and win the Nobel Prize?
Next Atomic Theory ?
Which one of you will develop a better theory and win the Nobel Prize?