1. The History of
ATOMIC THEORY
As told by Ms. Hummel

2. The Greeks: Empedocles 490-444 BC Leucippus and Democritus 460-420 BC
Plato BC
Aristotle BC

3. Empedocles ( BC)

4. Leucippus and Democritus (460 to 420 BC)
Founders of “atomism”
Democritus

5. Aristotle and Plato (427-322 BC)
Said that matter was continuous and could be broken down
Greek word “atomos” meaning “uncut” or “indivisible”
ATOM: smallest particle of an element that retains the
chemical identity of the element

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

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

8. 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 elements are different
from those of any other element.
3. Atoms of different elements can physically mix together
or 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.

11. Ben Franklin ( )
Proved electricity existed

12. Michael Faraday ( )
Suggested atomic structure
was related to electricity

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

14. Constructed cathode ray tube
Found rays bent towards a + charge

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

18. "Could anything at first sight seem
more impractical than a body which
is so small that its mass is an
insignificant fraction of the mass of
an atom of hydrogen which itself is
so small that a crowd of these atoms
equal in number to the population
of the whole world would be too small
to have been detected by any means
then known to science”
-- J.J. Thomson.

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

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

23. ONWARD TO THE
PROTON

24. 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

25. Notice that the rays are emitted from the POSITIVE electrode ( positive rays!)

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

27. “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!

28. 1903 Nagaoka
“Saturnian” model of atom with flat rings of electrons
revolving around a positively charged particle

29. 1909 Ernest Rutherford conducted the famous “gold foil” experiment
BUT THEN…..
1909 Ernest Rutherford conducted the famous
“gold foil” experiment

34. 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!

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

36. Neutral atoms have an equal number of electrons and protons
Mass of protons are 1836 time 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

37. James Chadwick (1932)

38. 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.

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

40. And then, of course, in 1930, Schrodinger viewed electrons as
continuous 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!

42. ………and this is only the beginning