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Atomic
Structure
History of Atomic Theory
Democritus (460 - 370 BC)
 Idea
of indivisible particles - atomos
Aristotle (460 - 370 BC)
 Dismissed
Democritus’ idea
 Earth/Wind/Water/Fire
 Fast
forward 2000 years….
John Dalton (1766-1844)
• No actual experiments
• Brought idea of an indivisible
particle back into fashion
John Dalton (1766-1844)
• His model:
Dalton’s Atomic Laws
1. Law of Conservation of Mass
2.
Law of Definite Proportions
JJ Thomson (1856-1940)
• Cathode Ray Tube Experiment
Cathode Ray Tube Experiment
Video Clip
https://www.youtube.com/watc
h?v=CsjLYLW_3G0
Cathode Ray Tube Experiment
Cathode Ray Tube Experiment
•Rays must have been made up
of very small, light, negatively
charged particles:
•Electrons.
Thomson’s Plum Pudding Model
+ charge sphere that has (- ) charged
electrons scattered inside, like “raisins” in
“plum pudding”.
Ernest Rutherford (1871-1937)
 Alpha
particles (+
charged) newly
discovered
 Theorized they should
pass through the positive
“dough” of the atom
Rutherford’s Gold Foil
Experiment
 https://www.youtube.co
m/watch?v=ecsgC1wSp5I
Rutherford’s Gold Foil
Experiment
Rutherford’s Gold Foil
Experiment
• Most particles passed through as expected
• 1% deflected back
• Conclusions:
• Atoms must be mostly empty space
• Must be a small, dense area of positive
charge in the center of the atom:
The Nucleus
Rutherford’s Planetary Model
•The electrons orbit
the nucleus.
Niels Bohr (1885-1962)
 Worked
in Rutherford’s lab
 Wondered
why electrons
are not attracted to the +
nucleus and cluster
around it
Niels Bohr (1885-1962)
 Observe
spectrum of colors
emitted by excited hydrogen
atoms:
Niels Bohr (1885-1962)
 Rather
than seeing a complete
spectrum of colors, he found only
individual, discrete lines
Niels Bohr (1885-1962)
 Bohr
hypothesized that atoms
had quantized energy levels
Bohr’s Energy Level Model
Electrons orbit in fixed circular energy levels around
the nucleus. Each circle has a different energy.
•Electrons can “jump” from one circle to the next

Bohr’s Energy Level Model
 He proposed the following:
1. Protons and neutrons are in the nucleus
2. Electrons can only be certain fixed distances
from the nucleus.
3. The electrons orbit the nucleus at fixed
(qunatized) energy levels.
4. The electrons must absorb or emit a fixed
amount of energy to travel between these
energy levels
Review
Who is the father of atomic theory?
Dalton
What was the first model of the atom?
Dalton’s Tiny Ball Model
What are Dalton’s 3 Laws?
Law of Conservation of Mass, Law of Constant
Composition, Law of Multiple Porportion
Review
How were Thomson’s and Dalton’s model
different?
Dalton’s model was 1 sphere that cannot be
divided, Thomson had the plum pudding
where electrons are randomly spread
throughout a positively charged sphere.
What did Thomson find out?
Atoms have electrons, they have a - charge
Review
What were Rutherford’s conclusions from the Gold
Foil Experiment?
 Atom has a positively charged nucleus
 electrons are outside the nucleus
 atoms are mostly empty
 Nucleus contains most of the mass.