Download Development of Atomic Theory Development of Atomic Theory

9/20/2016
Development of Atomic Theory
Development of Atomic Theory
Early Greek Theories
Early Greek Theories
• Democritus 400 BC
• Democritus 400 BC
– Known by peers as the laughing
philosopher
• Aristotle 350 BC
– taught matter was made of 4 “elements”;
earth, fire, water, air
– matter was continuous could be cut into
smaller and smaller pieces infinitely
– Elaborated the “atomist theory” originated
by his teacher, Leucippus
– proposed matter was made of indivisible
particles called “atomos” a Greek word
meaning “uncuttable”
– This theory persisted for ~ 2000 years
C. Eugene Bennett Department of Chemistry
Development of Atomic Theory
C. Eugene Bennett Department of Chemistry
Development of Atomic Theory
John Dalton 1805
Postulates of Dalton’s Theory
• British school teacher and chemist
•
• Recognized his experimental results were
inconsistent with Aristotle’s theory of
continuous matter
All matter consists of atoms; tiny, hard spheres that cannot be created or
destroyed
•
Atoms of an element are identical
•
atoms of different elements are distinct different
• Revived theory of Democritus
•
Compounds result from the chemical combination of a specific ratio of atoms
•
Atoms of one element cannot be converted into atoms of another element
– Coined the term “atom”
C. Eugene Bennett Department of Chemistry
J. J. Thompson 1904
C. Eugene Bennett Department of Chemistry
J. J. Thompson 1904
Dalton was wrong!
Results from cathode ray tube experiments
• ray was composed of “negative” particles with a very tiny mass
• Atoms are not hard, solid spheres, they have smaller parts
• these particles were the same regardless of the gas in tube
• Thompson concluded that cathode rays are made up of negatively
charged particles
– It did not matter what element was put into the cathode ray tube,
they all had negative particles that behaved the same way
– These negative particles that all atoms have are now called
ELECTRONS!
– Thompson knew that the positive bits had to be there, he just
never found them
C. Eugene Bennett Department of Chemistry
C. Eugene Bennett Department of Chemistry
1
9/20/2016
Plum pudding model of the atom
What’s in the box?
Is all the mass spread
throughout as in a box
of marshmallows”?
or is all the mass
concentrated in a dense
“ball-bearing”?
C. Eugene Bennett Department of Chemistry
Figure it out without opening (or shaking)
Shoot bullets randomly through the box. If it is
filled with marshmallows, all the bullets will go
straight through without (much) deflection
Figure it out without opening (or shaking)
If it contains a ball-bearing most the bullets will go straight through
without deflection---but not all
Very occasionally, a bullet will collide nearly head-on to the ball-bearing
and be deflected by a large angle
C. Eugene Bennett Department of Chemistry
Rutherford’s gold foil experiment
C. Eugene Bennett Department of Chemistry
Rutherford’s gold foil experiment
“You can learn a lot by throwing rocks.”
α-ray “bullets” were expected to scatter uniformly
C. Eugene Bennett Department of Chemistry
C. Eugene Bennett Department of Chemistry
2
9/20/2016
Rutherford’s Gold foil experiment
Interpretation of Rutherford’s results
Rutherford’s view of the atom
• Almost all of its mass and all the positive change
concentrated in a tiny nucleus at its center
• The vast majority of the atom consists of empty space
• “a fly in a cathedral”
C. Eugene Bennett Department of Chemistry
C. Eugene Bennett Department of Chemistry
Relative size of the nucleus
x10-5
Nucleus
99.97% of the mass
C. Eugene Bennett Department of Chemistry
3