Transcript History of the Atomic Theory

The student will learn:
1.
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Development of modern day Atomic theory
Early contributions of Democritus/Dalton
Discovery of electron: Thompson/Millikan
Discovery of nucleus: Rutherford/Chadwick
History of the Atomic Theory
460-370 BC
Democritus: small invisible particles called atomos.
How?
• 1766-1844
John Dalton Englishman
Dalton’s Atomic theory
• 1. element made up tiny particles called atoms
• 2. atoms of a given element are identical
• Ex) all carbon atoms have same physical & chemical properties
atoms of a different element are different in
some way
Ex) carbon atoms have different physical & chemical properties
than sulfur
Dalton’s Atomic Theory
• 3. Atoms of one element combine with atoms
of a different element to form Compounds.
* Law of Definite Proportion
» All samples of a compound contain the same ratio / or /
proportion of the elements
* Chemical Formulas
Dalton’s Atomic Theory
4.Chemical reactions involve reorganization of atoms
* atoms are not changed just rearranged
* atoms of one element cannot change into atoms of
another element.
“ cannot change copper into gold”
Dalton’s Atomic Theory
1766-1844
1st discovery of subatomic particles
Discovery of electrons
1897 J.J.Thomson : used a cathode ray tube
Discovery of electrons
Caused a vacuum in tube
Filled it with a gas
A piece of metal at each end + / Put electricity through it
Surface opposite the cathode+ glowed
Put magnets next to it = caused a deflection
Placed an object inside it cast a shadow
Placed a wheel inside and it moved
Thomson’s Plum Pudding
model of an atom.
Idea of the time….William Thomson aka…. Lord Kelvin
JJ Thomson 1897
Robert A. Millikan
American physicist- 1909
Charge of electron
1.592 x 10-19
coulomb
Oil drop
experiment:
suspended an oil
drop between two
charged plates.
If made of pieces……..
then should be able to break it into pieces……..
Discovery of Nucleus
Ernst Rutherford - 1911
Gold foil experiment:
Bombarded a piece of gold foil with alpha
particles which were considered positive.
Expected to go all the way through it.
Results of foil
experiment
if
Plum Pudding
model had
been correct.
Rutherford’s experiment.
Actual results.
p.72
“as if you had fired a 15 inch artillery shell at a
piece of tissue paper and it bounced back”.
Ernst Rutherford
Thought about for 2 years.
Rebounded positive 2He atom means Gold must
have a dense middle and be positive.
Named it nucleus and it must be positive.
Ernest Rutherford 1911
James Chadwick
discovery of neutron
• In 1913 he was in Germany at start of World
War I and would be interned in Ruhleben
P.O.W. camp just outside of Berlin.
• Hans Geiger and Ernest Rutherford
interceded for his release.
• It was his discovery of neutrons that led to
“nuclear fission”…….sleeping pills
Sir James Chadwick
English - 1913
• Atoms from Dalton
1800
• Electrons from J.J.Thomson
1897
• Nucleus from Rutherford
1911
• Neutron from Chadwick
1932
• Protons????
• Collaborative but assigned to Rutherford….because
needed something positive to make neutral.
What was discovered and who discovered it
in the following photo?
Who & what was discovered?