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8/8/2019 Atomic Models 1
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How do you generate a
picture of something youcant see?
Models of the Atom
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Democritus Atomic Philosophy
460 370 BC
Believed that atoms wereindivisible andindestructible.
Theory based in reason.
Lacked experimentalsupport and laterchallenged by Plato and Aristotle
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Daltons Billiard Ball Atom
1766 1844
English chemist, teacher
Studied the ratios in whichelements combine
Theory: All elements are composed of tiny indivisible particles called atoms.
Atoms of the same element are identical. The atoms of any one element are different
from those of any other element and can be distinguished by their mass. Atoms of different elements can physically mix together or can chemically combine in
simple whole-number ratios to form compounds. (law)
Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of
one element, however, are never changed into atoms of another element as a result of
a chemical reaction.
Atoms cannot be created, destroyed (law), or divided into smaller particles.
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Thomsons Plum Pudding model
1897
English physicist
Cathode ray experiment
Atoms contain some sortof charged subatomicparticle Implies positive & negative
Theory: positively chargedmedium with negativecharges embedded
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Rutherfords Nuclear Atomic Model
1911, English baron
Shot a stream of alpha
particles at a piece ofgold foil
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(Rutherford contd)
People thought that the particles would passthrough with a little deflection.
Surprise! Almost all of the particles passedthrough without any deflection.
A very small fraction were deflected at largeangles and some even bounced straight back.
Nuclear Atom theory Atoms are mostly empty space
Matter is concentrated at the center of atoms thenucleus and is very, VERY small
The nucleus has a positive charge
Electrons are distributed around the nucleus
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Sketch the models for
John Daltons Billiard Ball atom
J.J. Thomsons Plum Pudding atom
Lord Ernest Rutherfords Nuclear atom
How is each model similar to and different to
the others?
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Spectral lines (emission spectra)
When hit by light, elements give off
different spectra of light
Where is this light coming from?
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Bohrs Solar System Model
1913, Danish Physicist
Light emitted by excited atoms
is energy, emitted when
electrons gain or lose energy Electrons gain energy to move
away from nucleus and lose
energy when falling back
Electrons must orbit nucleusin fixed paths, each farther
from the nucleus than the last,
like planets around the sun http://www.mhhe.com/physsci/astronomy/applets/Bohr/applet_files/Bohr.html
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Heisenberg Uncertainty Principle
In order to see something, you need to
shine light on it and see the reflected light.
If you shine a light on an electron, it gainsenergy, jumps to a different level, and its
momentum changes.
Therefore, it is impossible to know both
the exact position and momentum of an
electron at any given time.
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Electron Cloud model (today)
If we cannot know the position and momentum of an
electron, then electrons cannot travel in orbits!
The best we can do is describe the areas of space in
which electrons are most likelyto be found known asorbitals
Based in quantum mechanics, developed
by Schrodinger, de Broglie, and others
Also known as the wave-mechanical orquantum-mechanical model
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Homework!
Summary of the five major atomic models
(Dalton, Thomson, Rutherford, Bohr, Cloud
Electron)
Sketch each model and label them by name
Beneath each sketch, explain why it was
proposed/how it was discovered
Describe the similarities and differences between
this model and the model before it Describe at least one shortcoming
(limitation/problem) with each of the first four
models