1. The History of Atomic Theory
Atomos: Not to Be Cut
The History of Atomic Theory

2. Atomic Models
This model of the atom may look familiar to you. This is the Bohr model. In this model, the nucleus is orbited by electrons, which are in different energy levels.
A model uses familiar ideas to explain unfamiliar facts observed in nature.
A model can be changed as new information is collected.

3. Ancient Times
The atomic model has changed throughout the centuries, starting in 400 BC, when it looked like a billiard ball →

4. Democritus
He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided?

5. Democritus
His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained.
This piece would be indivisible.
He named the smallest piece of matter “atomos,” meaning “not to be cut.”

6. The First Atomic Theory
To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes.
Atoms were infinite in number, always moving and capable of joining together.

7. This theory was ignored and forgotten for more than 2000 years!

8. Why was it forgotten?
The eminent philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately wrong) theory.

10. Dalton’s Model
In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.

11. Dalton’s Theory
All elements are composed of tiny indivisible particles called atoms
Atoms of same elements are identical. Atoms of different elements are different.
Atoms can chemically combine with others in simple whole-number ratios to form compounds
Chemical reactions occur when atoms are separated, joined or rearranged.

12. Jean Perrin ~ 1898
Used the gas tubes invented by Crookes that emitted cathode rays.
He collected the rays in a hollow metal cylinder that was basically a charge collector.
The ray was deflected towards the positive in an electric field!
The collected charge was negative!

13. Jean Perrin
Perrin also bounced a beam of these cathode rays off of a pinwheel type device.
The pinwheel spun around!
Conclusion…the cathode rays were NEGATIVELY CHARGED PARTICLES!!!

14. J.J. (Joseph John) Thomson
Conducted a series of hundreds of experiments with cathode ray tubes
He asked: If the gas in the tube is neutral, where are these negative particles coming from?

15. His experiments measured the speed of the cathode rays.
He also tried sending the ray through combinations of electrical and magnetic fields.
He used the speed and the degree of deflection to measure the charge to mass ratio for the cathode ray particles.

16. J.J. Thomson
His experiments suggested that the particles in cathode rays were over 1000 times lighter than the hydrogen atom (Atoms WERE NOT INDIVISIBLE!!!!)
The mass was the same whatever type of atom they came from.
Rays were composed of very light, negatively charged particles which were a universal building block of atoms. He called the particles "corpuscles", but later scientists preferred the name electron.

17. J. J. Thomson
Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom.
But he could never find them.

18. Thomson’s Plumb Pudding Model
He proposed a model of the atom that is sometimes called the “Plum Pudding” model.
Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

19. Ernest Rutherford
In 1908, performed his famous “Gold Foil Experiment” that had surprising results!
Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)

21. Rutherford’s Gold Foil Experiment
Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all.
Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.

22. Rutherford’s Conclusion
This could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material.
Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged “bullets.”
He called the center of the atom the “nucleus”
The nucleus is tiny compared to the
atom as a whole.

23. Bohr Model -1913
According to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus.

24. The Wave Model
Today’s atomic model is based on the principles of wave mechanics.
According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun.

25. The Wave Model
In fact, it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has.
According to the modern atomic model, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.

26. Murray Gell-Mann Won the Nobel Prize for Physics in 1969.
Formulated the a new model of elementary particles.
Had entered Yale University at age 15!
Appreciated for his wide range of interests and sense of humor.

27. Gell-Mann
As new sub-atomic particles were discovered, there was lots of confusion on how to organize and explain them.
Gell-Mann proposed the “strangeness number” and brought in Buddhist philosophy in describing the “eightfold way” to sort them into eight “families”.
There are particles still SMALLER than protons and neutrons – Three “quarks” make up each proton and neutron.

28. Gell-Mann He proposed that there are six “flavors” of quarks:Up
Down
Strange
Charm
Bottom
Top
Three quarks make up each proton and neutron!