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ATOMS
A Brief History
Democritus ~400 B.C.
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A philosopher
Good ideas but no scientific proof
No one believed him because Aristotle was
more popular (Earth, Air, Fire, Water)
First to suggest the existence of atoms (atomos)
Small indestructible pieces that make up everything
 Nothing is smaller than an atom
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Dalton ~1800 A.D.
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A teacher and chemist
“Father of the Modern Atomic Theory”
Transformed Democritus’ ideas into a scientific
theory
Used experimental methods
Studied how elements combine in chemical
reactions
Dalton (cont’d)
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Observations from experiments led him to the
following explanations:
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Everything is made of atoms. Nothing is smaller than an
atom.
Elements are a collection of same atoms. Atoms of an
element are different from atoms of another element.
Atoms can combine to form compounds in whole # ratios.
Chemical reaction = atoms being rearranged
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Atoms can join together or groups of atoms can separate
An atom cannot change into another atom
Thomson 1897
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A physicist who was studying how matter could
carry electricity.
Observation:
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When electricity passes through a few gas particles in
a tube, a glowing ray appears.
http://www.makingthemodernworld.org.uk/icons_of_invention/science/1880-1939/IC.026/
http://www.aip.org/history/electron/jjcrooke.htm
Thomson cont’d
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Hypothesis:
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The ray is a stream of negative particles because it
seems to originate from the negative electrode
(cathode) of the tube.
Experiment:
Pass electricity through different kinds of gas
particles.
 Subject the rays to a magnetic field and to an electric
field to see if anything changes.
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Thomson (cont’d)
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Results:
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All rays are deflected by both magnetic and electric fields.
All rays are deflected away from the negative part of the field.
There is a constant ratio relating the mass and charge of the
particles that made up the ray.
http://dev.physicslab.org/Document.aspx?doctype=3&filename=Magnetism_CathodeRays.xml
Thomson (cont’d)
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Conclusion:
Negative particles are a part of all matter
 Negative particles are the same in all matter
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What it means:
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All atoms contain ELECTRONS!
Follow-up:
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Another scientist used CRT to discover the
PROTONS in a similar way!
Rutherford 1911
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A chemist
Gold-Foil Experiment
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Thinnest possible layer of gold atoms
Theory: Large positive particles fired at the foil should be
only slightly deflected when reaching the detector on the
other side.
http://www.shsu.edu/~chm_tgc/sounds/sound.html
Rutherford cont’d
http://www.sparknotes.com/testprep/books/sat2/physics/chapter19section2.rhtml
http://www.dlt.ncssm.edu/TIGER/chem1.htm
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Results: Most went straight through! Some actually bounced
back!
Conclusion: Atom is mostly empty space. The positive
charge of the atom is concentrated in a tiny nucleus which
contains most of the mass.
Bohr 1913
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A physicist and student of Rutherford
Studied changes in atoms when interacting with
light
Observation: atoms change in specific ways when
interacting with light.
 Explanation: the electrons are actually organized
around the nucleus and exist in set circular paths like
planetary orbits.
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Quantum Model
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Newer theories now exist
Nucleus still has protons and neutrons and contains
most of the mass.
 Electrons are in highly complicated regions outside
the atom. All of these together make it look like a
simple electron cloud.
 Details will follow later this year!
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