1. Historical Atomic Theory
Developments in atomic theory from Democritus to Bohr

2. How do we know what is inside of something?
Open it up and look…
a baseball?
a golf ball?
an atom?
???

3. X X X Democritus Greek philosopher – ca. 400 BC
Atoms are particles in space. X
Atoms are indivisible.
Atoms are in motion.
Things differ because of different arrangements of atoms. X
Events are predictable. X
Everything is made up of atoms and “the void”.

4. Alchemists attempted to turn base metals into gold
Performed experiments
Recorded observations

5. At the age of 12!!!
John Dalton
English chemist / physicist / meteorologist / teacher
Tested/experimented
Purely Theoretical
Dalton’s model
Democritus’ ideas
All matter is made up of solid, indivisible spheres.
Atoms of the same element are identical.
Compounds are made up of different combinations of atoms.
Atoms can not be subdivided, created, or destroyed.
Chemical change is due to rearrangement of atoms.
Atoms are particles in space.
Atoms are indivisible.
Atoms are in motion.
Things differ because of different arrangements of atoms.
Events are predictable.
Everything is made up of atoms and the void.

6. Michael Faraday – 1820s-1840s+
Interested in the electrical nature of matter
Used electrolysis to decompose water
Thought that electricity was the “glue” that held atoms together.
Popularized terms:
electrode
cathode – negative electrode
anode – positive electrode
Discovered cathode rays.
beam of light that seemed to emanate from the cathode (traveled from the cathode  the anode)

7. Michael Faraday Interested in the electrical nature of matter
Cathode ray tube

8. William Crookes – ~1870 Developed Crookes’ tube
When turned on, the paddle wheel would move!!!
How?
 Light is made up of particles (w/ a mass).

9. William Crookes Light could be deflected by a magnetic field.
\Light must be charged particles…

10. J.J. Thomson – 1897 continued experiments involving CRTs
Expanded upon Crookes work…

11. J.J. Thomson Light could be repelled by negative charge.
Light could be attracted by positive charge.
\Light must be negatively charged particles…
called electrons
back to Thomson in a few minutes…

12. Eugene Goldstein – 1885 Developed Goldstein tube
A second beam of light, traveling in the opposite direction
positively charged particles
called them “canal rays”  protons (later named by Rutherford)

13. paper covered with barium platinocyanide
Wilhelm Röntgen – 1895
also working with CRTs / modified Crookes tubes
conducting experiments with faint luminescence
paper covered with barium platinocyanide
some “unknown” rays causing the paper to glow
called these unknown rays – x-rays

14. Henri Becquerel – 1896 following up on the work of Röntgen with
investigations of radiation
had inherited a supply of uranium salts (compounds) that phosphoresce on exposure to light
he set up an experiment where he wrapped some of these salts with photographic plates
found that the plates were already ‘fogged’…
radiation must be coming from the uranium atom
FIRST CLEAR INDICATION that the atom is not featureless, providing strong evidence to the existence of electrons.
radioactivity – named by Marie Curie (1898)
Becquerel, Marie & Pierre Curie shared a Nobel Prize (1903)

15. J.J. Thomson – 1897
further investigations changing the electrode material as well as the residual gas
regardless of changes:
repeatedly determined the charge to mass ratio (q/m):
all electrons, regardless of source, are identical

16. J.J. Thomson – 1897 Dalton’s model must be incorrect
atom is composed of positive material with negative particles (electrons) embedded in it
AKA Plum Pudding Model (1904)

17. Robert Millikan – 1909 American physicist Millikan oil drop experiment
was able to determine the charge on the droplet
with q/m ratio (Thomson) and the charge…
calculated the mass of an electron = 9.11 x grams

18. Ernest Rutherford – 1911
(1908) designed an experiment to test Thomson’s model of the atom
Gold foil experiment
used alpha (a) particles [product of radioactive decay]
small
dense
fast-moving
positive charge
(Actually 2 protons & 2 neutrons. Same as helium nucleus – but Rutherford did not know this…)

19. Ernest Rutherford – 1911

20. Ernest Rutherford – 1911
(1908) designed an experiment to test Thomson’s model of the atom
Gold foil experiment (click…)
A closer look (click…) More explanation???
 Atom contains a small, positively charged center (nucleus), surrounded by empty space with electrons

21. Light At the time of Rutherford:
electrons: particles
light: wave
But in the early 1900s, experiments began to produce evidence of:
electrons having wave-like properties
light having particle-like properties
Photoelectric effect
explained by Einstein
Hot body radiation
explained by Planck

22. Niels Bohr – 1913
Electrons can only exist in a certain path (orbit) of a certain energy.
An electron does not emit (give off) energy when in a ground state orbit.
Energy is emitted when an e- goes from an excited state (higher energy level) to a lower energy level.
Atoms emit (or absorb) energy only in specific amounts (quantas).
The energy emitted (or absorbed) is equal to the difference in the energy between energy levels.
(Angular momentum of electron is quantized)

23. Niels Bohr – 1913
Bright line spectra explained

24. Review
Dalton  Thomson 
Rutherford  Bohr