Transcript Review Models of the Atom

The Experiment
• To test this he designed and experiment directing ‘alpha’
particles toward a thin metal foil.
– The foil was coated with a substance that produced flashes
when it was hit by an alpha particle.
Some a particles
are scattered
Source of
a particles
Beam of
a particles
Screen to detect
scattered a particles
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56
Most particles
pass straight
through foil
Thin metal foil
Appling the Results to the Models
Alpha particles
Nucleus
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Plum-pudding atom
Nuclear atom
Models of the Atom
"In science, a wrong theory can be valuable and better than no theory at all."
- Sir William L. Bragg
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Dalton’s
Greek model
model
(400
(1803)
B.C.)
Thomson’s plum-pudding
model (1897)
Bohr’s model
(1913)
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125
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Rutherford’s model
(1909)
Charge-cloud model
(present)
Models of the Atom
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Dalton’s
model
Greek model
(1803)
(400 B.C.)
1803 John Dalton
pictures atoms as
tiny, indestructible
particles, with no
internal structure.
1800
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Thomson’s plum-pudding
model (1897)
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Rutherford’s model
(1909)
1897 J.J. Thomson, a British
1911 New Zealander
scientist, discovers the electron,
leading to his "plum-pudding"
model. He pictures electrons
embedded in a sphere of
positive electric charge.
Ernest Rutherford states
that an atom has a dense,
positively charged nucleus.
Electrons move randomly in
the space around the nucleus.
1805 ..................... 1895
1900
1905
1910
1904 Hantaro Nagaoka, a
Japanese physicist, suggests
that an atom has a central
nucleus. Electrons move in
orbits like the rings around Saturn.
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125
1915
Bohr’s model
(1913)
1926 Erwin Schrödinger
1913 In Niels Bohr's
model, the electrons move
in spherical orbits at fixed
distances from the nucleus.
1920
1925
Charge-cloud model
(present)
1930
develops mathematical
equations to describe the
motion of electrons in
atoms. His work leads to
the electron cloud model.
1935
1940
1945
1924 Frenchman Louis
1932 James
de Broglie proposes that
moving particles like electrons
have some properties of waves.
Within a few years evidence is
collected to support his idea.
Chadwick, a British
physicist, confirms the
existence of neutrons,
which have no charge.
Atomic nuclei contain
neutrons and positively
charged protons.
Bohr Model
Neils Bohr
Planetary
model
After Rutherford’s discovery, Bohr
proposed that electrons travel in definite
orbits around the nucleus.
• Bohr’s contributions to the understanding of
atomic structure:
1. Electrons can occupy only certain regions of space,
called orbits.
2. Orbits closer to the nucleus are more stable —
they are at lower energy levels.
3. Electrons can move from one orbit to another by
absorbing or emitting energy, giving rise to
characteristic spectra.
• Bohr’s model could not explain
the spectra of atoms heavier
than hydrogen.
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.