The Development of Atomic Models
• Democritus was a preSocratic Greek
philosopher (born
around 460 BC).
• Democritus was
originator of the belief
that all matter is made
up of various
imperishable, indivisible
elements which he
called "atomos", from
which we get the
English word atom.
•According to legend,
Democritus was supposed to
be mad because he laughed
at everything, and so he was
sent to Hippocrates to be
cured. Hippocrates pointed
out that he was not mad, but,
instead, had a happy
disposition. That is why
Democritus is sometimes
called the laughing
philosopher.
BB - Model
Dalton’s Model
More to come
Plum Pudding Model
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Proposed by J. J. Thomson
(1856 - 1940), the discoverer of
the electron in 1897.
The plum pudding model was
proposed in March, 1904 before
the discovery of the atomic
nucleus.
In this model, the atom is
composed of electrons
surrounded by a soup of positive
charge to balance the electron's
negative charge, like plums
surrounded by pudding. The
electrons were thought to be
positioned throughout the atom.
Electrons could move like letters
in alphabet soup
Instead of a soup, the atom was
also sometimes said to have had
a cloud of positive charge.
Thomson's model was compared
(though not by Thomson) to a
British treat called plum pudding,
hence the name. It has also been
called the chocolate chip cookie
model, but only by those who have
not read Thomson's original paper
Nuclear Model
• The Gold foil experiment
or the Rutherford
experiment was an
experiment done by
Ernest Rutherford (1871 1937) in 1909. This
experiment discovered the
nucleus.
• Led to the downfall of the
plum pudding model of
the atom.
• Alpha particles (positive
particles--Helium Nuclei)
were shot at gold foil.
• Particles passed through
the gold foil. A few shot
back.
• Conclusions:
1. Atom is mostly empty space
2. Dense center called the nucleus
3. Electrons were stuck surrounding
the nucleus.
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Planetary Model
• Introduced by Niels Bohr, a
Danish physicist (1885 1962), in 1913.
• Because of its simplicity,
the Bohr model is still
commonly taught to
introduce students to
quantum mechanics.
• The Bohr model depicts
the atom as a small,
positively charged nucleus
surrounded by waves of
electrons in orbit — similar
in structure to the solar
system, but with
electrostatic forces
providing attraction, rather
than gravity.
"The opposite of a correct statement is a
false statement. But the opposite of a
profound truth may well be another
profound truth." Niels Bohr
Quantum Mechanical Model
• Erwin Schrödinger (August
12, 1887 – January 4, 1961)
• An Austrian physicist,
achieved fame for his
contributions to quantum
mechanics, especially the
Schrödinger equation, for
which he received the Nobel
Prize in 1933.
• This model is based on
probability
• Where are you going to find
and electron 90% of the time.
• Atom is viewed as a fuzzy
cloud.
• Schrödinger equations create
electron clouds (orbitals) with
specific shapes.
Main Points For “Atoms” Video
• What is the key to understanding atomic structure?
• The discovery of what particle is associated with the
Crook’s Tube?
• What did Rutherford expect to happen in the gold foil
experiment?
• What was Rutherford’s genius?
• What conclusions did Rutherford draw from the Gold
foil experiment?
• How much smaller is the nucleus than the electron
cloud?
• What determines the shape of the electron cloud?