1. HISTORY OF THE ATOMIC MODEL ATOMIC THEORY
Mrs. Gibson
Physical Science

2. Democritus 400BC Proposed all objects have a smallest part/piece
Named this smallest piece – atom
Atomos means indivisible, not to be cut
Atom – small hard particle, made of same material, but in different sizes, shapes, and colors, indefinite in number, always moving, join together.
Uses observations to prove ideas
Consider what you may know about Aristotle. Four elements – Fire, Water, Air, Earth… Scientific notions based only on what could be observed, whales and turtles were the same kind of animal because they both lived in water. Realize that Democritus developed arguments also based on observation and reasoning. No real data or experimentation. Lack of Technology to aid his efforts, yet he still is on the right track. Much of Science follows this same general timeline.

3. Uses scientific reasoning to prove ideas
John Dalton 1803
Dalton in 1803 had available more technology certainly than Democritus, consider that electricity was available, but light bulbs would take another 50+ years. Dalton is able to conduct further experiments, considering the greater amount of knowledge about substances and able to support conclusions with more scientific fact. However, even his conclusions were not perfect and very general.
Uses scientific reasoning to prove ideas

4. J. J. Thomson 1897 Discovers that the atom is divisible.
Theory is called Plum Pudding Model
Atom is positive material with negative particles mixed in.
Discovers Electron  called it a corpuscle
Discovers electron by passing electric current through a gas
Gas is neutral, but gives off negative charges.
Reasoned: If there is a negative charge, then there must be a positive substance to make the gas neutral.
J.J. more technological advancement at his hands was able to conduct further experiments and used indirect evidence to support the things he could not see directly, based on other scientific facts. What was thought indivisible, was now experimentally proven divisible.

5. J.J. with his Cathode Ray Tube
Plum Pudding Model

6. Ernest Rutherford 1908 Gold Foil Experiment
Discovers center of atom, Nucleus, contains positive Protons
Atom has positive center with negative charges (electrons) scattered outside in electron cloud.
Most of atom is empty space
More Technology and Experimentation provide further insight into the makings of the atom

7. Gold atoms and alpha particles (path) up close
GOLD FOIL EXPERIMENT
Gold atoms and alpha particles (path) up close
Gold Foil Experiment – alpha particles (He nuclei) from a radioactive source are emitted and either pass through or bounce off of a sheet of gold foil. The phosphorus detecting screen around the foil shows the emission of light when the particles strike it. Most of the particles pass easily through the gold foil sheet. Rutherford reasons that the atom then is mostly empty space. However because some of the particles bounce back, he reasons that there must be a “solid” middle and that it must be positive since it repels the positive alpha particle. If it was negative then the particles would be pulled in by the electromagnetic force. See the small diagram to the right. Therefore, Thomson’s negative charges must be free floating in the empty space.
Most of the particles pass through the gold foil
-some particles bounce back- must be a “solid” middle and it must be positive since it repels the other alpha particles
-negative particles must be floating in the empty space

8. Rutherford’s Model Empty Space Positive Nucleus Protons Electrons
Rutherford also hypothesized about a neutral particle, but could not experimentally prove it.
Protons
Electrons

9. Comedic Pause
Two atoms were walking down the street when one stops and shouts
“Wait! Stop! I lost an electron.”
“You did? Are you sure?” says the second atom frantically.
“Yes,” says the first “I’m POSITIVE!”
Wait for it...
Laughter and Applause

10. Niels Bohr 1913 Changed Rutherford’s model
Show that electrons are not just scattered, but travel around nucleus in orbitals called energy levels.
The Bohr Model is used most to represent atomic structure

11. I wonder if they know that MY model is better than their model.
Bohr Model
I wonder if they know that MY model is better than their model.
Notice the nucleus is in the middle. The energy levels are positioned on orbits encircling the nucleus. The current Bohr Model has been adapted due to new information from the Wave Model Theory.

12. James Chadwick 1932 Discovers Neutron
In 1932 the Neutron was discovered. What key development occurred in 1945 that changed the world and was directly related to processes associated with the Neutron? Atomic Bomb. Think about the timing.

13. Chadwick Finds Neutron by…
READ EXCERPT… taken from Nuclear Chemistry, Discovery of the Neutron (1932) Dr. Frank Settle
When James Chadwick reported to Lord Rutherford on the Joliot-Curies’ results, Lord Rutherford exclaimed, "I do not believe it!" Chadwick immediately repeated the experiments at the Cavendish Laboratory in Cambridge, England. He not only bombarded the hydrogen atoms in paraffin with the beryllium emissions, but also used helium, nitrogen, and other elements as targets. By comparing the energies of recoiling charged particles from different targets, he proved that the beryllium emissions contained a neutral component with a mass approximately equal to that of the proton. He called it the neutron in a paper published in the February 17, 1932, issue of Nature. In 1935, Sir James Chadwick received the Nobel Prize in physics for this work. You can read his lecture as he received his Nobel prize. It is interesting to note that the Joliot-Curies’ misinterpretation of their results cost them the Nobel Prize. (Not to worry; in 1935, they received the Nobel Prize in chemistry for their discovery of artificial radioactivity.)
Chadwick concluded that the energy of the protons could be explained by the presence of a neutral particle comparable in mass to that of a proton.

14. Wave Model Current Model Theory, based on wave mechanics
Cannot determine exact location of electrons, but can determine probable locations based on calculations.
Energy Levels fill
2 - 1st
8- 2nd
8-18 in 3rd
Further Technology, provides a more detailed look each day at what the atom has to offer. We even know now that the Protons and Neutrons are made up of another subatomic particle the “quark”.

15. Let’s Explore Electron Configuration…
SIDEBAR
Electron Applet
For more information try the tutorial: Elements as Atoms

16. Summary What was the Theory of… Who discovered the…
Democritus?
Dalton?
Thomson?
Rutherford?
Bohr?
Chadwick?
Wave?
Who discovered the…
Electron?
Proton?
Nucleus?
Neutron?
What allows the scientists to develop/discover each new piece of the atomic model?