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Origins of the atomic theory
1. Origins of the Atomic
Theory
Physical Science
2009-2010
psquires
2. The Development of the
Atomic Theory
♦Democritus
and Dalton:
♦atomic theory
3. Democritus
♦Greek philosopher ~ 300 BC
♦Limit to “smallness”
♦All matter consists of tiny,
indestructible particles called
atoms
♦Atomos – indestructible
4. John Dalton
First serious atomic theory
♦English scientist
♦Studied the properties of gases
♦“Reinvented” the idea of atoms
♦Published in 1803
5. Dalton’s atomic theory - 1803
1. Elements are composed
of tiny, discrete, particles
called atoms.
6. Dalton’s atomic theory - 1803
2. Atoms are indivisible
and indestructible and do
not change their identity
during reactions.
7. Dalton’s atomic theory - 1803
3. Atoms of the same
element are identical in
mass and chemical and
physical properties. Atoms
of different elements are
different.
8. Dalton’s atomic theory - 1803
4. Atoms combine to form
compounds in simple,
whole-number ratios.
Law of Definite Proportions
9. Dalton’s atomic theory - 1803
5. Atoms combine in
different ratios to make
two or more compounds.
Law of Multiple Proportions
11. J. J. Thomson
♦Cathode rays - cathode ray tube
♦Attracted to positive electrode
♦Thought they might be atoms
♦Had same charge to mass ratio
regardless of metal in the cathode
♦Particle must be common to all
matter, a subatomic particle
12. Cathode Ray Tube
It was also used by J. J. Thomson
Cathode Anode
High voltage +
13. That particle was
called the …
The
The electron electron
The Electron
The electron
The electron
Discovered in 1897
The electron
By J. J. Thompson
14. J. J. Thomson
If that were the case, then the
electron would be much smaller
than the smallest atom,
… showing for the first time
that matter is made up of
particles smaller than atoms.
Thomson tried to measure the
fundamental charge on the electron.
15. The Development of the
Atomic Theory
♦Rutherford: “Gold Foil
Experiment”
21. Rutherford’s Nuclear Atom
Alpha particles were repelled by…
… a small, dense, positively
charged nucleus.
Almost all the mass of an
atom is in the nucleus.
Electrons are located outside the
nucleus.
22. Niels Bohr
♦ Electrons travel in fixed orbits around the atom’s
nucleus.
♦ Bohr also described the way atoms emit radiation
by suggesting that when an electron jumps from
an outer orbit to an inner one, that it emits light.
♦ Later other physicists expanded his theory into
quantum mechanics.
♦ This theory explains the structure and actions of
complex atoms.