atomic theory very good
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When do scientific theories change?
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Democritus develops the idea of atoms
460 B.C.
he pounded up materials until he had reduced
them to smaller and smaller particles which
he called
ATOMA
(greek for indivisible)
Were Democritus tiny particles really undividable?
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The Greeks believed
that all matter was made from 4 elements
Aristotle (384-322 BC)Fire is NOT matter! What form of energy is fire?
WRONG
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1803 John Dalton
EXPERIMENTAL OBSERVATIONS
Combined elements to form compounds
Observed that the ratio of elements in acompound was always the same
element + element = compound
Magnesium + oxygen = magnesium oxide
10 grams + 6.58 grams = 16.58 grams
1000 grams + 658 grams = 1,658 grams
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1803 John Dalton
Theory (model)
All matter is made of particles that
cant be divided ATOMS
An ATOM is like a solid sphere
Elements are made of 1 type of atom
Compounds contain more than 1 kind
of atom
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1897 JJ Thompson
EXPERIMENTAL OBSERVATIONS
Negatively charged beam attracted to a
positive plate (magnet)
ELECTRON
Sealed tube filled with gas
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JJ THOMSON
Theory (model)
Negative(-) particles are a part of the atom
Since the atom as a whole, is neutral, there must also be
positive (+) matter within the atom
Chocolate Chip IceCream Model
Chips are negativeparticles
Ice cream is positivematter
1897
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1911 Ernest Rutherford
oversaw Geiger and Marsden carrying out his
famous gold foil experiment.
they fired Helium nuclei (also called alpha
particles) at a very, very thin piece of gold
foil(only a few atoms thick).
They predicted that the alpha particles
would pass right through the foil.
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1911 Ernest Rutherford
Experimental Observations
They found that most
of the alpha particles
passed through the foil.
However, about 1 in10,000 were deflectedhttp://regentsprep.org/Regents/physics/phys05/catomodel/ruther.htm
http://regentsprep.org/Regents/physics/phys05/catomodel/ruther.htmhttp://regentsprep.org/Regents/physics/phys05/catomodel/ruther.htm -
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1911 Ernest Rutherford
Theory (model)
Alpha particle deflection
was caused by a dense
nucleus
the positive chargein an
atom are concentrated in
the nucleus
Predicted result
Actual result
What do the blue circles on the diagram represent?
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1913 Niels Bohr
studied under Rutherford at the
Victoria University in Manchester.
Bohr refined Rutherford's idea by
adding that the electrons were in
orbits.like planets orbiting the
sun.
Link to Niels Bohrs paper on Atoms and Molecules
http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.html
http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.htmlhttp://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.htmlhttp://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.htmlhttp://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.htmlhttp://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.htmlhttp://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Bohr/Bohr-1913a.html -
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Bohr Model
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Modified
Bohr Model of a
helium atom
++
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proton
electron neutron
shell
(energy level)
Electrons travel
around the nucleuslike planets aroundthe sun.
In 1932, James Chadwick confirms the existence of neutrons on in the nucleus of atoms.
What is different about this model compared to the previous slide?
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1926 Erwin Schrodinger
Schrodingers Cat: a thoughtexperiment.
Use mathematics to describethe motion and locations ofelectrons.
Electron Cloud Model of theatom was developed as aresult of Schrodingers workwith mathematical probability.
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Electron Cloud Model
This model shows theprobability (likelihood) offinding an electron.
The cloud model represents
a sort of history of where anelectron has been and whereit is going.
Imagine if the electron left
a trace of where it was. Thecollection of traces wouldbegin to resemble a cloud.
Where are we most likely to find
an electron within this atom?
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Scientific Theory May Change
As scientists have made new observations of matter,their understanding of the atom changed.
Scientists must be open to new evidence. A scientific
theory does not represent truth but instead our bestunderstanding.
Does the statement above mean that the scientistswho have studied the atom were wrong?
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MORE ON ATOMS
Actual picture of atoms. Kinda looks like
solid spheres from this perspective.
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Energy LevelsElectrons that are closer to
the nucleus have less
energy.
When electrons are in their
lowest energy levelsthey
are in their ground state
When an electron absorbsenough energy, it may move
to a higher energy
levelexcited state
When an electronreturns to its ground
state
light may be released
from the atom
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Isotopes
The same atomswith different
numbers of
neutrons.
How can you
determine the
mass number of
an atom?
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ATOMIC STRUCTURE
What is the atomic number of
the atom shown above?
What is its mass number?
17
35
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ATOMIC STRUCTURE
Particle
proton
neutron
electron
Charge
+ charge
- charge
No charge
1
1
1/1837
Mass
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SUMMARY
1. The Atomic Numberof an atom =number of
protons in the nucleus.
2. The Mass Numberof an atom =number of
Protons + Neutrons in the nucleus.
3. The number of Protons =Number of Electrons.
4. Electrons orbit the nucleus in shells(levels).
5. Electrons can move between levels as energy isabsorbed or emitted.
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