1. The History of
ATOMIC THEORY

2. The Greeks
Empedocles ( BC)
The four elements: fire, water, wind, and earth.

3. Democritus (460 to 420 BC)
Founder of “atomism”. He believed atoms were indivisible and indestructible.
Greek word “atomos” meaning “uncut” or “indivisible”
Democritus

4. Aristotle and Plato (427-322 BC)
Said that matter was continuous and could be broken down into atoms.
ATOM: smallest particle of an element that retains the
chemical identity of the element

5. …and for the next 2000 years atomic theory
was based on abstract thinking,
not experimentation
until . . .

6. John Dalton ( )
English school teacher
Work completed
Presented Dalton’s
Atomic Theory

7. Dalton’s Atomic Theory
1. All elements are composed of submicroscopic,
indivisible particles called atoms.
2. Atoms of the same element are identical.
The atoms of any one element are different
from those of any other element.
3. Atoms of different elements can physically mix together
to form a mixture. Or they can chemically combine with one another in simple whole-number ratios to form compounds.
4. Chemical reactions occur when atoms are separated,
joined or rearranged. However, atoms of one element
are never changed into atoms of another element as a
result of a chemical reaction.

8. Sticky Tape
Cut two pieces of tape and move the ends close together. What are your observations?
Put one piece on the desk sticky side facing down. Put the other piece on top of the first piece sticky side down. Remove both pieces and separate them. Hold the ends close together. What are your observations?
Conclusion: Atoms contain charged particles.

9. Ben Franklin ( )
Michael Faraday ( )
Suggested atomic structure
was related to electricity
Proved electricity existed

10. Discovery of the Electron
Joseph John (JJ) Thomson ( )

11. Constructed cathode ray tube
Found rays bent towards a + charge and bent away from a negative plate.

12. Concluded rays were negatively charged particles
Called the particles “electrons” after the Greek word
“elektron” meaning amber

14. Mass and Charge of the Electron
Robert Millikan ( )
1909 American Physicist from
University of Chicago
Determined charge and mass
of an electron through the famous
“oil drop” experiment

17. Charge of an electron: 1.60 x 10 -19 coulomb
Mass of an electron: x g

18. ONWARD TO THE
PROTON

19. History of discovery of the proton is rather vague:
Eugene Goldstein (1886) -
Used cathode ray tube but with Ne atoms
with electrons removed
Called “positive rays” because they went from
positive electrode to negative
J.J. Thomson (1906)
Studied positive ray deflection
When hydrogen was used, he got the smallest particles
Called the particles PROTONS

20. Neutral atoms have an equal number of electrons and protons
Mass of protons are 1836 time (about 2000 times) greater than the mass of electrons
So……..
How are the electrons and protons arranged in an atom?
We had our theories……..

21. “cookie dough” or “plum pudding” model
J.J. Thomson
“cookie dough” or “plum pudding” model
Protons and electrons embedded together in atom
like chocolate chips in cookie dough!

22. 1903 Nagaoka
“Saturnian” model of atom with flat rings of electrons
revolving around a positively charged particle

23. 1909 Ernest Rutherford conducted the famous “gold foil” experiment
BUT THEN…..
1909 Ernest Rutherford conducted the famous
“gold foil” experiment

26. This led to a new model for the atom……..
A dense nucleus of positive charge with the electrons circling around it
Size scale: if the nucleus of the atom was the size of a tennis ball, the atom would have a diameter over 1 mile. The nearest electron would be .25 mi from the nucleus! If the nucleus was the size of a quarter, the diameter of the atom would be about the length of this room.

27. So this is where we’ve come so far…..

28. Neutral atoms have an equal number of electrons and protons
Mass of protons are 1836 time (about 2000 times) greater than the mass of electrons
Did not explain why the mass of He was 4 times
the mass of hydrogen
ONWARD TO THE
NEUTRON

29. James Chadwick (1932)

30. But that’s not all, folks.
In 1922 a gentleman by the name of
Niels Bohr
developed an explanation of atomic structure
that underlies the regularities
of the periodic table.

31. His atomic model had atoms built of successive orbital SHELLS of atoms

32. mathematical model of the atom
And then, of course, in 1930, Schrodinger viewed ELECTRONS AS CONTINIOUS CLOUDS and introduced “wave mechanics” as a
mathematical model of the atom
The mathematical equation is:
…but we’ll talk about Bohr and Schrodinger later!

33. ………and this is only the beginning