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Platinum and Nickel
How did we get here?
Copper Atoms
Blue- tin
Red- silicon
Green- lead
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we see atoms
like the ones
shown on this
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Carbon atoms
ATOMIC THEORY
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Cat
Joke
Scientists
Ancient
Greeks
Democritus
&
Leucippus
J.J.
Thomson
Ernest
Rutherford
Summary
Video
John
Dalton
Niels Bohr
Atom
Review
Click on scientist to guide you through the PowerPoint.
All green boxes are links. Use them to guide you!
Cat Joke!
Scientist
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Ancient Greeks
(450 BC)
• Believed all matter
was made of:
– Earth
– Fire
– Water
– Air
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Cat Joke!
Cat Joke!
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Democritus & Leucippus (300 BC)
Experiment
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Experimental
Conclusions
New
Model
Experiment
• They did not run any experiments.
• Atoms were an invention of their minds
Democritus &
Leucippus
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Experimental Conclusions
• Atoms are the smallest particles of matter
• Defines the atom as the smallest particle
• Explains the existence of elements
Democritus &
Leucippus
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Model
Name:
Atom the indivisible particle
Description:
Matter is made up of
particles so small and
indestructible that they
cannot be broken down into
smaller units- particles were
called “atomos”, which is
Greek from “indivisible”
Democritus &
Leucippus
Model
Shortcomings
Model Shortcomings
• No scientific view of the atom only a
conceptual definition
• Does not talk about subatomic particles
(electrons, protons, neutrons)
Democritus &
Leucippus
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Democritus &
Leucippus
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John Dalton (1800)
Dalton Fun
Facts 
Scroll to them
Experiment
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Experimental
Conclusions
New
Model
Experiment
• Dalton ran experiments with different types of
gases and through deductive reasoning and
experimentation he came to several
conclusions.
Dalton
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Experimental Conclusions
1. Pure Elements consist of particles called
atoms.
2. Atoms of an element are all the same for that
element. That means gold is gold and oxygen
is oxygen down to the last atom.
3. Atoms of different elements can be told apart
by their atomic weights.
4. Atoms of elements unite to form chemical
compounds.
5. Atoms can neither be created or destroyed in
chemical reaction.
Dalton
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Dalton’s Model
Name:
The Solid Sphere Model
(Clay Ball Model)
Description:
– Atoms are seen as solid,
indestructible spheres (like
billiard balls)
Dalton
Model
Shortcomings
Model Shortcomings
• Atoms are not indivisible (b/c
they are made up of subatomic
particles)
• Atoms can change from one
element to another by
radioactive decay (not by
chemical reactions).
• Not all atoms of the same
element have exactly the same
mass (isotopes).
Dalton
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Dalton
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J.J. Thomson (1850)
Experiment
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Experimental
Conclusions
New
Model
Experiment
Experiment
Animation
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Summarized Experiment
Detailed explanation
of cathode ray tube
Voltage
Cathode
(-)
•
•
•
•
•
Click here
anode
(+)
Empty glass tube (vacuum) connected to a metal cathode and anode.
High voltage is applied to the tube.
This caused the atoms in the cathode to get excited.
Electrons stream off the cathode, producing a cathode ray.
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The cathode ray traveled toward the anode
Summarized Experiment
Voltage
Cathode
(-)
anode
(+)
Click here
When a negative charge was brought toward the cathode ray, it
caused the cathode ray to move away from the negative charge.
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Summarized Experiment
Voltage
+
Cathode
(-)
anode
(+)
Click here
When a positive charge was brought toward the cathode ray, it
caused the cathode ray to move toward the positive charge.
Thomson
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Experimental Conclusion
• Discovered the electron
– Electrons have negative charges
• Electrons are evenly distributed in a positive
sphere.
Thomson
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Thomson’s Model
Name:
Plum Pudding Model
Description:
• Atoms are solid spheres made
up of a solid positive mass with
tiny negative particles
embedded in the positive core.
• Like a chocolate chip cookie!
– Cookie is positive
– Chocolate chips are negative
electrons
Thomson
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Real Plum Pudding! Gross!!
Thomson
Model
Shortcomings
Model Shortcomings
• Does not explain the existence of electrons
outside the nucleus
• Does not explain the role of electrons in
bonding
• Does not talk about neutrons, protons, or the
nucleus
Thomson
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Thomson
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Ernest Rutherford (1905)
Experiment
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Experimental
Conclusions
New
Model
Experiment
Experiment
Animation
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Summarized Experiment
• Sent alpha particles (+2 charged Helium atoms) at a thin layer of
gold foil
• He expected 100% of the particles to go through to screen
• 90% went through the foil to screen, but 10% were deflected
• He was surprised and wrote that this result was comparable with
shooting a bullet at a piece of paper and having the shell reflected
back.
• The plum pudding model could not be correct.
Rutherford
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Experimental Conclusion
• Atoms are mostly empty space
• Atoms have a dense center with a positive
charge called a nucleus
• Electrons move around the nucleus at a
distance
Detailed explanation of
gold foil experiment
Rutherford
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Rutherford’s Model
Name:
Nuclear atom
Description:
• Positively charged nucleus in
center of atom
• Electrons moving around
nucleus at a distance
Nucleus
Rutherford
Model
Shortcomings
Electrons
Model Shortcomings
• It does not explain: how negatively charged
electrons keep from crashing into the positivelycharged nucleus.
Rutherford
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Rutherford
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Niels Bohr (1920)
Experiment
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Experimental
Conclusions
New
Model
Experiment
Experiment
Animation
An important experiment
was the study of the
emission of light by excited
hydrogen atoms.
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Experiment Summary
Bohr
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• When atoms receive energy, electrons move way from
nucleus (higher energy level)
• When atoms release energy, electrons move toward
nucleus (lower energy level) and release light
Click for examples
Energy
Released
Energy
Added
e-
Absorbs energy
Releases energy
e-
Experimental Conclusion
• Electrons can only exist on definite energy
levels that are specific distances from the
nucleus.
Bohr
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Bohr’s Model
Name:
Planetary Model
Model
Animation
Description:
• Positively charged nucleus in
center of atom
• Electrons moving around
nucleus at specific energy levels
(or distances from nucleus)
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Bohr
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Summary of Atomic Theory
Summary
Animation
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Atom
Review
Atom Review
• Mass number = #protons + # neutrons
• Atomic number = #protons= element
• Charge= #protons - # electrons
Mass Number
23
Atomic Number
11
+1
Charge
Na
Nuclear symbol
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Atom Review
• Mass number = #protons + #neutrons
• Average atomic mass = percent abundances
of all the isotopes of an element
12
C
13
C
14
C
The average atomic mass is on the periodic table:
C
12.01
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Sample Average Atomic Mass Calculation
• Determine the average atomic mass of Ar if
the isotopes are: 5% 79Ar and 95% 80Ar
• Step 1: convert % to decimal
• 0.05 and 0.95
• Step 2: multiply decimal by mass number
• 0.05 X 79 = 3.95
• 0.95 x 80 = 76
• Step 3: add values together
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• 3.95 + 76 = 79.95
• Step 4: check that the value matches the
value shown under Ar on the periodic table