1. The History of the Atom

2. Learning Target: Identify major contributions to the modern theory of atomic structure brought forth by John Dalton, J.J. Thomson, Robert Millikan, Ernest Rutherford, Niels Bohr, and James Chadwick.

3. John Dalton Born 1766 Did most work from 1808-1827 England
First to develop a theory for the structure of an atom - Based on five major principles

4. Dalton’s Atomic Theory
Five Major Principles:
Elements are made of indivisible atoms
All atoms of the same element have the same properties and the same mass
Compounds are made of atoms of different elements combined together
Chemical reactions involve the reorganization of those atoms
Atoms cannot be created nor destroyed

5. Dalton’s Atomic Theory
The first and second principles: Elements are made of indivisible atoms AND All atoms of the same element have the same properties and the same mass
“Atomos”
His thought was that each element existed as a simple spherical substance, different in physical and chemical properties from any other element
This is his MODEL of the ATOM

6. Dalton’s Atomic Theory
The third principle: Compounds are made of atoms of different elements combined together
When Dalton was able to separate a substance, he called it a compound. Remember, elements could not be separated into smaller parts.
Unlike a mixture, compounds are chemically combined and can not be separated without a chemical change.

7. Dalton’s Atomic Theory
The third principle… what else does it tell us?
When a particular compound is created, Dalton stated that it must be created the exact same way each time. Compound #31 must always be made of three empty spheres (oxygen) with one sphere with a cross (sulphur).
This idea is called the Law of Definite Proportions – when elements combine, they do so in a ratio of whole numbers.

8. Dalton’s Atomic Theory
The third principle… can different compounds be made of the same elements?
Yes
Take a look at compounds #26 and #27 
One represents what Dalton believed to be nitrous oxide and the other nitric acid.
These are an excellent example of the Law of Multiple Proportions, which states that if two elements form more than one compound between them, then the ratios of the different compounds must be different from each other.

9. Dalton’s Atomic Theory
The fourth principle: Chemical reactions involve the reorganization of those atoms

10. Dalton’s Atomic Theory
The fifth principle: Atoms cannot be created nor destroyed
In all chemical reactions the total mass of the original chemicals equals the total mass of the created chemicals. This is known as the Law of Conservation of Mass.

11. Dalton’s Atomic Theory
It’s flawed! Are atoms only spheres? Or, is there something else inside?

12. Dalton’s Atomic Theory
Another flaw – Look at compound #21, Dalton’s version of water; that’s one oxygen with one hydrogen. Isn’t water made with two hydrogen atoms?

13. Dalton’s Atomic Theory
So why is it still studied? Despite these major flaws, Dalton opened the door to enormous understandings of atoms, compounds, and reactions that we still use today. And, three major laws that are also still important – Law of Definite Proportions, Law of Multiple Proportions, and Law of Conservation of Mass.

14. Lived 1856 – 1940 England Did work from 1884 - 1914
J.J. Thomson
Lived 1856 – 1940 England
Did work from

15. Thomson’s Experiment
Thomson was fascinated by the idea that electricity could cause a trapped gas to glow. He called his device a cathode ray tube, or Crooke’s tube; these are still used today as standard fluorescent bulbs, neon lights, and spectroscope tubes, to name a few current models.

16. Thomson’s Experiment
It’s not the cathode ray tube that’s fascinating, or useful to the development of the atom, however. It’s the fact that magnets held against the tube could deflect (or attract) the light beam inside.

17. Thomson’s Atomic Model
The Plum Pudding Model
The electrons were thought to be positioned throughout the atom, but the atom was also was said to have had a "cloud" of positive charge.
Thomson won the Nobel prize for the discovery of the electron in 1906.

18. Robert Millikan Lived 1868 - 1953 University of Chicago!
Experimenting with the input of Harvey Fletcher

19. Millikan’s Experiment of 1909

20. Millikan’s Results
Millikan’s Oil Drop Experiment, published in 1913, helped the atom in two ways.

21. Millikan’s Results First, it showed the charge on the electron…
1.6x10-19 coulombs.
And second, it showed the mass of the electron was 1/2000th of a proton.

22. Millikan’s Results
And second, it showed the mass of the electron was 1/2000th of a proton. Thomson had already determined the charge-to-mass ratio for the electron, but the actual values were unknown. Millikan was able to calculate it.

23. Ernest Rutherford Lived 1871 - 1937 New Zealand origin, England lab
One of Thomson’s students

24. Rutherford’s Experiment
Alpha Particle 4 +2 2 He
Rutherford’s Gold Foil Experiment, in , predicted that all particles would go straight through the atom described by the Plum Pudding Model.

25. Rutherford’s Experiment
That’s not what happened, though!
Some particles went through the foil while others were deflected and still others bounced back.

26. Rutherford’s Atomic Model
The Planetary Model
Rutherford proved that there was a positively charged, very dense, very heavy center, called the nucleus. While the rest of the atom was empty space.
He is credited with the discovery of the proton.

27. Niels Bohr Lived 1885 - 1962 1913 published theory on atomic structure
Denmark

28. Bohr’s Experiment
Bohr’s Hydrogen Energy Level Experiment used a spectroscope to separate the colors of a heated hydrogen gas. What could cause specific colors of light to be created, rather than a full continuous spectrum?

29. Bohr’s Experiment
Specific colors of light have specific wavelengths of specific energy. This led Bohr to theorize that there are levels, called energy levels, around the atom’s nucleus for electrons to travel between

30. Bohr’s Experiment
Bohr realized that electrons must absorb energy as the hydrogen gas is heated. When the electron absorbs the energy, it moves to a higher energy level.
The electron eventually releases that energy, as a wavelength of color, falling to a lower energy level.

31. Bohr’s Atomic Model The Orbital Model
Electrons travel in orbits, now called energy levels, around the atom's nucleus. The variable “n” represents the energy levels.

32. Bohr’s Atomic Model
Bohr’s Orbital Model is still not the current model of the atom. It did, however, determine some important information about atoms.
The chemical properties of each element can largely be determined by the number of electrons in the outer orbits of its atoms.
Bohr’s work became the basis for quantum mechanics which is the current model of the atom.

33. James Chadwick
Lived
Major discovery: 1932
England

34. Chadwick’s Experiment
Alpha Particle Bombardment
Alpha particles (the same particles that Rutherford used) were used to bombard a sample and it was discovered that neutral particles were released that changed the mass of the atom.

35. Chadwick’s Atomic Model
The neutron was the last subatomic particle discovered.
Chadwick won the Nobel Prize for his discovery of the neutron in 1935.

36. Erwin Schrödinger
Lived
Germany

37. Schrödinger’s Atomic Model
Louis de Broglie had determined that electrons moved in wave-like motion.

38. Schrödinger’s Atomic Model
This meant that electrons were not moving around the nucleus in orbits, as described by Bohr, but, rather, in clouds.
The Cloud Model

39. Schrödinger’s Atomic Model
Joining this idea with the theories of the scientists before him, Schrödinger developed the Schrödinger equation. The Schrödinger equation and it’s use of quantum mechanics is the current model of the atom.