Transcript Document
History of the Development of
Atomic Theory
Your first task…
• In each envelope you will find 7 yellow
pictures illustrating the various models that
have been used to explain the structure of the
atom
• Using clues from the pictures, put the pictures
in chronological order (which model do you
think came first, second, etc.)
Your second task…
• Using the clues from the 7 blue scientist cards,
match up the models (yellow cards) with the
appropriate scientist card
• When you feel that you have made the
appropriate matches, make sure that your
cards are in chronological order
(Hint: There are dates on the blue cards!)
Democritus, 400 B.C.
• Elements consist of tiny,
solid particles that can
not be subdivided.
• He called these particles
atomos, meaning
“uncuttable.”
John Dalton, 1805
• 1. All matter is made of tiny
indivisible particles called atoms.
• 2. Atoms of the same element are
identical; those of different elements
are different.
• 3. Atoms of different elements
combine in whole number ratios to
form compounds
• 4. Chemical reactions involve the
rearrangement of atoms. No new
atoms are created or destroyed.
Legos are Similar to Atoms
H2
H
H
H
O
+
H2
H
H
O2
O
H 2O
O
H 2O
H
H
O
H
Lego's can be taken apart and built into many different things.
Atoms can be rearranged into different substances.
Joseph John (J.J.)
Thompson, 1904
• Discovered the electron.
• Atoms contain small, negatively
charged particles.
• The electrons are evenly embedded
throughout a positively charged
sphere (like chocolate chips in a ball
of cookie dough)
• Known as the “Plum Pudding Model”
A Cathode Ray Tube
Source of
Electrical
Potential
Stream of negative
particles (electrons)
Metal Plate
Gas-filled
glass tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58
Metal plate
Thomson’s Experiment
ON
-
OFF
voltage
source
+
Passing an electric current makes a beam appear
to move from the negative to the positive end
Thomson’s Experiment
ON
-
OFF
voltage
source
+
Thomson’s Experiment
ON
-
OFF
voltage
source
+
+
-
By adding an electric field…
he found that the moving pieces were negative.
He concluded that these negative pieces must be
suspended in a positive sphere
Ernest Rutherford,
1911
• Almost all of the mass of an
atom- and all its positive chargesare concentrated in a small,
central atomic nucleus
surrounded by electrons.
• Used a “gold foil” experiment to
come to his conclusions
Results of foil experiment if plum-pudding
had been correct.
Electrons scattered
throughout
-
+
-
positive
charges
+
+
+
+
-
-
+
+
+
-
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57
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Rutherford’s Apparatus
Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemistry.
beam of alpha particles
radioactive
substance
circular ZnS - coated
fluorescent screen
gold foil
Niels Bohr, 1913
• Electrons travel in fixed orbits
around the atom’s nucleus (like
the planets around the sun).
James Chadwick, 1932
• A student of Rutherford,
concluded that the nucleus
contained positive protons and
neutral neutrons
• Discovered the neutron
Electron Cloud Model, Current
• Electrons do not follow fixed orbits
but tend to occur more frequently
in certain areas around the nucleus
at any given time.
• These areas are called electron
clouds.