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Models of the Atom
"In science, a wrong theory can be valuable and better
than no theory at all."
Sir William L. Bragg
Flaws in the Bohr model
Increasing energy
of orbits
n=3
e-
n=2
e-
n=1
ee-
e-
e-
e-
e-
e-
e-
eA photon is emitted
with energy E = hf
The Bohr model of the atom, like many ideas in the history of science,
was at first prompted by and later partially disproved by experimentation.
Models of the Atom
So far the theories amounted to...
e
e
+
e
+
e
+
+
e
+e
+e
e
+ e + e
Dalton’s
Greek model
model
(400
(1803)
B.C.)
Thomson’s plum-pudding
model (1897)
-
-
- +
Rutherford’s model
(1909)
Bohr’s model
(1913)
The problem was…
If we pass light WAVES through a
diffraction grating, we obtain a
diffraction pattern
VISIBLE LIGHT
ELECTRONS were believed to be PARTICLES.
Passing electron particles through a
diffraction grating produces a diffraction
pattern
ELECTRONS
Electrons as Waves
Evidence: DIFFRACTION PATTERNS
VISIBLE LIGHT
ELECTRONS
Models of the Atom
Until then the theories had amounted to...
e
e
+
e
+
e
+
+
e
+e
+e
e
+ e + e
Dalton’s
Greek model
model
(400
(1803)
B.C.)
Thomson’s plum-pudding
model (1897)
-
-
- +
Rutherford’s model
(1909)
Bohr’s model
(1913)
The wavelike properties of
electrons led to the charge cloud
model
Charge-cloud model
(present)
Wave – particle duality
DE BROGLIE WAVELENGTH
Prince Louis de Broglie - 1932
De Broglie
discovered that all
particles with
momentum have
an associated
wavelength.

h
h

p
mv
Erwin Schrödinger (1926)
Quantum mechanics
• electrons can only exist in
specified energy states
Electron cloud model
• orbital: region around the
nucleus where e- are
likely to be found
Quantum Mechanics
• Heisenberg Uncertainty Principle
– Impossible to know both the velocity and
position of an electron at the same time
g
Microscope
Electron
Werner Heisenberg
~1926
Erwin Schrödinger (1926)
Electron Cloud Model (orbital)
dots represent probability of finding an
-
Quantum Mechanical Model
Niels Bohr &
Albert Einstein
Modern atomic theory describes the electronic
structure of the atom as the probability of finding
electrons within certain regions of space (orbitals).
Current View
• The atom is mostly empty space
• Two regions
– Nucleus
• protons and neutrons
– Electron cloud
• region where you might find an electron