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Chapter 5Where is the electron found?
Different types of spectra
Continuous spectra from incandescent light bulb
Line emission spectrum of Hydrogen
Line emission spectrum of Sodium
Line emission spectrum of Mercury
Neils Bohr
• Bohr treated the
electron as only a
particle, like dust.
This electron orbits
the nucleus in
discrete paths.
• ”planetary model”
“Anyone who is not shocked by quantum
theory has not understood it.”
• Electrons in lowest energy
states= ground state
• When energy is added, electrons
can be promoted to higher energy
states= excited.
• Light (as photons) is emitted
when an electron returns to the
ground state= emission spectrum
• ground state: e- in
lowest possible
energy
• excited state: egains energy,
moving to a state
where atom has
more energy
– in an excited state,
e- releases specific
quantity of energy as
it “falls” back to
ground state.
•An electron in a hydrogen atom can
move between only certain energy
states, shown as n = 1 to n = 7.
Bohr’s model explained the
Hydrogen atom well, but not others.
Further experiments clouded the
atomic model….
Quantum Model
• Modern model of the atom
• Electrons can be found in
orbitals or “electron clouds”
• Quantum model, or modern
model, can also be called the
“electron cloud model”
Evolution of the atomic model
Evolution of the atomic model
Bohr model of the atom= electrons reside in
“orbits” around the nucleus, only certain
“orbits” are allowed, like RUNGS on a ladder.
Evolution of the atomic model
Quantum model of the atom= electrons are in
an “electron cloud”. Their exact location is not
known.
<>
http://htwins.net/scale2/
48 Iron Atoms on a
Cu surface
“Quantum Corral”
Circa 1985
Scanning Tunnelling
Microscopy
Image
“The ripples in the ring
of atoms are the
density distribution of
a particular set of
quantum states of the
corral.”
http://www.almaden.ib
m.com/vis/stm/corral.h
tml#stm7
2. Where are the electrons?
We don’t know, for sure.
As soon as we try to measure the
location of the electron, we are
DISTURBING the system. Also, if an
electron is a wave and a particle,
How do we measure it?
We are forced to speak in
PROBABILITIES instead of
CERTAINTIES.
Einstein explains the
Photoelectric effect,1905
•
•
When light is shone
on clean metal
surfaces in vacuum,
electricity is
produced. Only
certain types
(wavelengths) of
light create
electricity.
ELECTRONS have
particle behavior.
Louis deBroglie ,1924
Electrons could be considered as
waves confined to the space
around a nucleus
http://www.spaceandmotion.com/quantum-theory-de-broglie-quotes.htm
Thus, electrons AND
light display the
WAVE-PARTICLE
DUALITY OF LIGHT
AND ELECTRONS
••••••••
So, light and electrons can be a
particle AND a wave?
• Light and electrons behave like waves as
they diffract and interfere (Louis De Broglie)
• Light and electrons behave like particles in
the photoelectric effect and emission spectra.
• So the compromise is that they are BOTH!
Summary videos
• http://www.youtube.com/watch?v=xqNSQ
3OQMGI
• http://www.youtube.com/watch?v=YYBCNQnYNM
Erwin Schrodinger
• Made calculations of electron
behavior describing it as a wave
function, ψ (psi).
• Electrons are in probable
locations known as electron
clouds.
• “Schrodinger’s cat” paradox
• Paradox=a statement that may
be true but seems to say 2
opposite things. Something full
of contradictions.
Demo grating on overhead,
spectrometers with gas discharge
tube, flame tests
• In 1802 Spectral lines from the sun were
see by William Wollaston.A German
optician names Joseph von Fraunhofer
observed and cataloged the lines in 1814,
measuring the positions of the lines. In
1850 Gustav Kirchoff investigated further,
with the help of Robert Bunsen (of the
Bunsen burner).
• Excellent interactive web site for more information
about quantum theory
http://www.colorado.edu/physics/2000/
Click on “Science Trek” and “Quantum Atom”
Areas of high probability for
electrons are orbitals
• Each orbital can only have 2 electrons
• What do the orbitals look like?
Different types of spectra
Emission spectrum of Sodium
• Heisenberg
uncertainty principle:
you can never know
exactly where the
electron is if you know
how fast it is moving.
WERNER HEISENBERG (1901 - 1976