1) Early Greek philosopher Democritus proposed that all matter is made of tiny, indivisible particles called atoms. 2) In the early 1800s, English scientist John Dalton developed the first serious atomic theory, proposing that elements are made of atoms and atoms of the same element have the same properties. 3) J.J. Thomson's discovery of the electron in 1897 showed that atoms contain smaller subatomic particles, challenging the idea that atoms are indivisible.

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

10. The Development of the
Atomic Theory
♦Thomson: CRT’s and the electron

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”

16. The Gold Foil Experiment
Top
View
Side View

17. The Gold Foil Experiment
Gold foil Fluorescent
detector
ZnS
Alpha
particle
source
All of this was in a
vacuum chamber.

18. The Gold Foil Experiment
Most of the
α particles
went…
…straight through
the gold foil,
undeflected.
The gold is mostly “empty space.”

19. Alpha Particles
Alpha particles are helium nuclei.
Two protons
+ and
+ two neutrons.
The alpha particle is positively charged.

20. Gold Foil Experiment: Results
α source
+
Small, dense,
positively charged
nucleus of gold

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.

23. Bohr
diagram