1. Atomic Structure Timeline
History of Atomic Theory

2. Essential Questions
What does it mean when my science teacher says stuff cannot be created or destroyed?
What is matter made up of? And who figured it out?

3. Evolution of Modern Atomic Theory

4. Democritus (400 B.C.)
Proposed that matter was composed of tiny indivisible particles
Not based on experimental data
Greek: atomos

5. The Early Atom
Aristotle succeeded Democritus and did not believe in atoms. Instead, he thought that all matter was continuous. It was his theory that was accepted for the next 2000 years.

6. Alchemy (next 2000 years) Mixture of science and mysticism.
Lab procedures were developed, but alchemists did not perform controlled experiments like true scientists.

7. Lavoisier 1777
French chemist, who is considered the founder of modern chemistry.
Lavoisier clarified the concept of an element as a simple substance that could not be broken down by any known method of chemical analysis
He devised the Law of Conservation of Mass

8. Basic Laws of Matter
Law of Conservation of Mass- mass is neither created nor destroyed during ordinary chemical reactions or physical changes.
CH4 + 2O2 → 2H2O + CO2
16g + 64g → 36g g
Antoine Lavoisier
stated this about 1785

9. Alka Seltzer in Water Ziploc bag Alka seltzer tablet Water
Using the reaction between the tablet and the water, prove that the Law of Conservation of matter is true.

10. Antoine Lavoisier and his wife, Marie-Anne
"It took them only an instant to cut off that head, and a hundred years may not produce another like it." Joseph-Louis Lagrange

11. Proust (1794) Proust stated the Law of Definite Proportions.
The law states that the ratio of elements in a compound is always the same.    

12. Basic Laws of Matter
Law of Definite Proportions – no matter how much salt you have, it is always 39.34% Na and 60.66% Cl by mass.
Example: Sodium chloride always contains 39.34% Na and 60.66% Cl by mass.
2NaCl → 2Na Cl2
100g → 39.34g g
116.88g → ? ?
Joseph Louis Proust
stated this in 1794.

13. I was a school teacher at the age of 12!
John Dalton (1808)
Dalton add his own - the Law of multiple proportions
He proposed a theory to summarize and explain these three laws
I was a school teacher at the age of 12!

14. John Dalton proposed this in 1803.
Basic Laws of Matter
Law of Multiple Proportions- Two or more elements can combine to form different compounds in whole-number ratios.
Example
John Dalton proposed this in 1803.

15. John Dalton Dalton’s Four Postulates
1. Elements are composed of small indivisible particles called atoms.
2. Atoms of the same element are identical. Atoms of different elements are different.
3. Atoms of different elements combine together in simple whole number ratios to create compounds.
4. In a chemical reaction, atoms are rearranged, but not changed.

16. John Dalton’s Elements

17. Was Dalton right?
Elements are composed of small indivisible particles called atoms.
Subatomic particles – electrons, protons, neutrons, and more
Atoms of the same element are identical. Atoms of different elements are different.
No, isotopes are atoms that have the same number of protons but a different number of neutrons

18. Was Dalton right?
Atoms of different elements combine together in simple whole number ratios to create a compound.
Yes! He was right!
In a chemical reaction, atoms are rearranged, but not changed.
Yes! He was right!

19. Essential Questions
So, Dalton figured out the atom, how did we figure out what the inside of an atom looks like?

20. How Big (Small) Is the Nucleus of an Atom?
If the total atom was the size of a football field, then the nucleus would be the size of an apple in the middle of the field.

21. The width of a human hair = 1,000,000 carbon atoms across
How Small is an Atom?
The width of a human hair = 1,000,000 carbon atoms across
One bucket full of water contains more atoms than there are bucketfuls of water in the Atlantic ocean

22. Discovery of the Electron
In the late 1870’s many experiments were performed in which electric current was passed through gases at low pressures due to the fact that gases at atmospheric pressure don’t conduct electricity well.
These experiments were carried out in glass tubes called cathode-ray tubes or Crookes tubes for the man that developed them.

23. Crookes Tube CRT

24. J. J. Thomson (1903) Cathode Ray Tube Experiments
beam of negative particles
Discovered particles smaller than the atom!

25. J.J. Thomson (1903)
Proved that the atom was divisible and that all atoms contain electrons.
This contradicted Dalton’s Atomic Theory.
This allowed a new model of the atom.

26. J. J. Thomson (1903) Plum-pudding Model
positive sphere (pudding) with negative electrons (plums) dispersed throughout

27. Robert Millikan (1910’s) American physicist
Determined the mass and charge of an electron
Oil drop experiment

28. Oil Drop Experiment
Millikan dropped negatively charged microscopic oil particles into a chamber containing metallic plates and viewed them with a microscope.
By applying voltage to the metallic plates, Millikan created an electric field.
He was able to suspend the oil droplets by adjusting the electric field to the appropriate strength and direction to overcome gravity.

29. Oil Drop Experiment
Knowing the mass of the droplets and the strength of the electric field necessary to suspend them, he was able to calculate the charge of the electron.
He noticed that the charge was always a whole-number multiple of X10-19 Coulombs.

30. Ernest Rutherford (1911) Gold Foil Experiment Discovered the nucleus
dense, positive charge in the center of the atom
Nuclear Model

31. Gold Foil Experiment

32. Gold Foil Experiment
As expected, most of the alpha particles passed straight through with little or no deflection.
However, 1/8000 of the positively charged alpha particles were deflected, some back at the source.

33. Gold Foil Experiment

34. Gold Foil Experiment
From this experiment, Rutherford discovered that there must be a very densely packed positively charged bundle of matter within the atom which caused the deflections.
He called this positive bundle the nucleus.
He tried this experiment with other metals and found the same results.

35. Gold Foil Experiment
The volume of the nucleus was very small compared to the volume of the atom.
Therefore, most of the atom was composed of empty space.

36. “It was about as believable as if you had fired a 15-inch shell at a piece of tissue paper, and it came back and hit you.”
-Ernest Rutherford

37. Ernest Rutherford (1911) Nuclear Model

38. Evolution of Modern Atomic Theory
Almost there! - - - - - - - + -

39. Niels Bohr (1913) Met with J.J. Thomson but didn’t impress him
Worked with Rutherford and liked his model of the atom
Incorporated idea of quantum mechanics into the Rutherford model

40. Niels Bohr (1913)
Introduced the idea of electrons traveling around the nucleus in orbits or energy levels.
Like the planets, there are multiple orbits like concentric circles around the nucleus
Planetary Model

41. Niels Bohr (1913) He used Bright-Line Spectrum
tried to explain presence of specific colors in hydrogen’s spectrum
Built upon Max Planck’s concepts of quantized energy

42. Erwin Schrödinger (1926) Treats electrons as waves
Tells us the probability of finding an electron at any given location at any given moment
Electron cloud model
Atomic orbital: region around the nucleus where electrons are likely to be found

43. Electron Cloud Model (orbital)
Erwin Schrödinger (1926)
Electron Cloud Model (orbital)
dots represent probability of finding an e- not actual electrons

44. James Chadwick (1932)
Studied under Rutherford and built upon his model
Discovered neutrons
neutral particles in the nucleus of an atom

45. He projected alpha particles (from a decaying polonium) towards beryllium target.
The particles released are uncharged and have the same mass as protons.
He called them neutrons.

46. revision of Rutherford’s Nuclear Model
James Chadwick (1932)
Neutron Model
revision of Rutherford’s Nuclear Model

47. Models of the Atom Timeline
Let’s recap with a video (10:52)

48. Dogs teaching Atoms