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Chapter 4
Atomic Structure
Early Atomic Theory
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Greeks: The world is made of two things,
empty space and “atoms”. Atoms are the
smallest possible stuff.
Aristotle: Matter is continuous, it isn’t made
from smaller stuff.
The Law of Definite Proportions
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Joseph Proust:
Specific substances
always contain
elements in the same
ratio by mass.
Hydrogen and Oxygen in Water.
1g
2.7 g
35.5 g
1g
Atom = indivisible sphere
Antoine Lavoisier
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Discovered that
when a chemical
change occurred
in a closed
system, the mass
before the
chemical change
equaled the mass
after the chemical
change.
Atom = indivisible sphere
Dalton’s Atomic Theory
Elements are made of tiny particles called
atoms.
 All atoms of a given element are identical.
 All atoms of a given element are different from
the atoms of other elements.
 Atoms can combine with each (in small whole
number ratios) other to form different
compounds. (ex: NO, NO2, N2O)
 Atoms are indivisible.
Atoms can be divided. Electrons are
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transferred during chemical reactions,
and nuclear reactions are possible, though rare.
Atom = indivisible sphere
JJ Thomson
Thomson presented three hypotheses
about cathode rays based on his 1897
experiments:
 He discovered that particles could come
from atoms which had a negative charge,
and bent in response to a magnet.
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http://www.aip.org/history/electron/jj1897.htm
Atom =
sphere containing negative charges
Robert Millikan
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Oil Drop Experiment
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Millikin sprayed oil through an atomizer, as it exits the
chamber it becomes negatively charged.
The bottom plate is negative, and repels the negatively
charged oil droplet, so it hovers above the bottom plate if
there is enough negative charge to repel the drop.
By adjusting the amount of charge, Millikan found that
the charges on the doplets were a multiple of the same
number.
y = mx ; where the multiple was “m”.
“m” is the amount of charge = to one charge. “X” was the
independent variable in this experiment.
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Atom =
sphere containing negative charges
Henry Mosely
studied X-rays from anodes of different
metals.
 The wavelength (color) produced by these
X-rays depends on the number of protons.
 The number of protons = atomic number.
 Z = atomic number.
 Atomic number determines the identity of
the atom.
Atom = sphere containing
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both positive and negative charges
James Chadwick and Walter Bothe
both discovered the proton.

In 1932 Chadwick observed that beryllium, Be,
when hit with alpha particles, released an
unknown radiation that in turn ejected
protons from the nuclei of various substances.
Chadwick interpreted this radiation as being
composed of particles of mass approximately
equal to that of the proton, but without electrical
charge--neutrons.
Atom = sphere containing positive,
negative, and neutral charges
++
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Be
X
Li
X-1
Isotopes:
J.J. Thomson observed two kinds of neon
atoms. neon-20 and neon-21
 Both neons are chemically identical, but
their masses differ.

Atom =
sphere containing positive,
negative charges, and neutral particles.
Henry Moseley 1913
Using X-rays, Moseley discovered that the
wavelength of X-rays is characteristic of
the metal used as an anode.
 The number of protons = the identity of the
element.

Atom =
sphere containing positive,
negative charges, and some number
of neutral particles, depending upon
the isotope.
Summary
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Democritus: Atom
Aristotle: Continuous Matter
Proust: the Law of Definite Composition
Lavosier: the Law of Conservation of Matter
Dalton: “Modern” Atomic Theory
JJ Thompson: Discovery of Electrons
Millikin: Charge/Mass ratio of electrons
Moseley: Discovery of Protons/Atomic Number
Chadwick and Bothe: Discovery of Neutrons
Rutherford’s Team:
Neils Bohr, Hans Geiger, Ernest Marsden.
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Gold Foil Experiment.
most of the alpha
particles went straight
through, some were
deflected by a “large” mass.
The PROTON was discovered.
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Atom =
sphere containing positive,
negative charges, and some number
of neutral particles, depending upon
the isotope. The positive charges were
located in the nucleus.
The Nuclear Atom
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Gold Foil Experiment.
+
most particles
go straight through.
The Nuclear Atom
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Gold Foil Experiment.
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most particles
go straight through.
Some are deflected
by the nucleus
Summary
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Democritus: Atom
Aristotle: Continuous Matter
Proust: the Law of Definite Composition
Lavosier: the Law of Conservation of Matter
Dalton: “Modern” Atomic Theory
JJ Thompson: Discovery of Electrons
Millikin: Charge/Mass ratio of electrons
Moseley: Discovery of Protons/Atomic Number
Chadwick and Bothe: Discovery of Neutrons
Rutherford: The Nuclear Atom
Henri Becquerel
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Found that matter containing uranium
exposed sealed photographic film.
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Time-Line of Discovery
http://www.chemheritage.org/explore/matt
er-time.html
 Other web sites
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http://www.slcc.edu/schools/hum_sci/physics/whatis/biography/dalton.html
http://www.chemheritage.org/EducationalServices/chemach/ppt/jd.html
http://antoine.frostburg.edu/chem/senese/101/atoms/dalton.shtml
http://www.fact-index.com/l/la/law_of_definite_proportions.html