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5.61 Fall 2007
Lecture #4
page 1
The ATOM of NIELS BOHR
Niels Bohr, a Danish physicist who established the Copenhagen school.
(a) Assumptions underlying the Bohr atom
(1)
Atoms can exist in stable “states” without radiating. The states have
discrete energies
E
n
,
n
= 1, 2, 3,.
.., where
= 1 is the lowest energy
state (the most negative, relative to the dissociated atom at zero
energy),
= 2 is the next lowest energy state, etc. The number “
” is
an integer, a
quantum number
, that labels the state.
(2)
Transitions between states can be made with the absorption or
Δ
E
emission of a photon of frequency
ν
where
=
.
h
1
h
or
Absorption
Emission
2
These two assumptions “explain” the discrete spectrum of atomic vapor
emission. Each line in the spectrum corresponds to a transition between two
particular levels.
This is the birth of modern spectroscopy
.
h
(3)
Angular momentum is quantized:
!
=
n
"
where
"
=
2
π
Angular momentum
L =
!
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 Spring '11
 Moh
 Atom, pH

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