Quantum Theory
Electromagnetic Spectrum
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H
E
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Y
Visible spectrum
Violet
400 nm
Blue
Green
500 nm
L
O
W
Yellow Orange Red
600 nm
700 nm
White
Light
g rays
X-rays Ultraviolet
Infrared
Microwave
Radio waves
Radar
TV Short
FM Wave
10-2nm 10-1nm 100nm 101nm 102nm 103nm 10-3cm 10-2cm 10-1cm 100cm 101cm
1cm
101m
102m
Long
Wave
103m
104m
Wavelength, l
1019Hz 1018Hz 1017Hz 1016Hz 1015Hz
1014Hz 1013Hz 1012Hz 1011Hz 1010Hz 109Hz 100 MHz 10 MHz 1 MHz 100 KHz
Frequency, n
Electromagnetic spectrum
http://www.youtube.com/watch?v=bjOGNVH3D4Y
Davis, Frey, Sarquis, Sarquis, Modern Chemistry 2006, page 98
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Photoelectric Effect
• Light is a form of energy
• Light can hit a metal surface and cause the
metal to emit electrons
– The photoelectric effect
• Light travels in waves
• Light at any frequency can hit a metal surface
and cause the metal to emit electrons
Photoelectric Effect
http://phet.colorado.edu/en/simulation/photoelectric
Photoelectric Effect
Light
Nucleus
Metal
A metal did not emit electrons when certain frequencies of light hit it
When red light strikes a metal surface, no electrons are ejected.
Photoelectric Effect
Light
Electron
Nucleus
Metal
There was a minimum frequency of light needed to get a metal to emit electrons
When green light strikes a metal surface, electrons are ejected.
Planck’s Explanation
• If electromagnetic radiation acted
as a wave, then it would emit
energy continuously
• Instead, electromagnetic radiation
is emitted in small specific
amounts
– Called quanta
• AKA: Things come in chunks
• Quantum: the minimum energy
that can be lost or gained by an
atom
Continuous vs. Quantized
A
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 330
B
Einstein’s Explanation
• Built off of Planck’s ideas
– Agreed that electromagnetic
radiation would emit energy
continuously if it acted as a wave
– Agreed that electromagnetic
radiation emits quanta
Einstein’s Explanation
• Noticed that electromagnetic
radiation is sometimes
continuous and sometimes
quantized
– Proposed a dual wave-particle
nature of electromagnetic
radiation
– Light has wavelike properties
– Light can be thought of as a
stream of particles
Einstein’s Explanation
• Defined the term photon
– A particle of electromagnetic
radiation (light) that has no mass
and carries a quantum (bundle) of
energy
• In order for the photoelectric
effect to occur
– The metal is struck by photons
– Each photon must carry a certain
amount of energy in order to
knock an electron loose from the
metal
5.1
The Bohr Model
– Bohr proposed that an electron is found only
in specific circular paths, or orbits, around the
nucleus.
Bohr Model of Hydrogen
Nucleus
e
e
Possible electron orbits
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 331
5.1
The Bohr Model
• Each possible electron orbit in Bohr’s model has a
fixed energy.
– The fixed energies an electron can have are called
energy levels.
– A quantum of energy is the amount of energy required to
move an electron from one energy level to another
energy level.
5.1
The Bohr Model
• Like the rungs of the
strange ladder, the
energy levels in an atom
are not equally spaced.
• The higher the energy
level occupied by an
electron, the less
energy it takes to move
from that energy level to
the next higher energy
level.
5.1
The Quantum Mechanical Model
• The propeller blade has the same probability of
being anywhere in the blurry region, but you
cannot tell its location at any instant. The
electron cloud of an atom can be compared to a
spinning airplane propeller.
5.1
The Quantum Mechanical Model
• In the quantum mechanical model, the probability
of finding an electron within a certain volume of
space surrounding the nucleus can be
represented as a fuzzy cloud. The cloud is more
dense where the probability of finding the electron
is high.
5.1
Atomic Orbitals
• An atomic orbital is often thought of as a region of
space in which there is a high probability of finding
an electron.
– Each energy sublevel corresponds to an orbital of a
different shape, which describes where the electron is
likely to be found.
5.1
Atomic Orbitals
• Different atomic orbitals are denoted by letters.
The s orbitals are spherical, and p orbitals are
dumbbell-shaped.
5.1
Atomic Orbitals
• Four of the five d orbitals have the same shape but
different orientations in space.
Vocabulary
• Orbital
– The space around a nucleus that has a high probability of
finding an electron
– Simply a probability graph of where we can find an
electron
• Quantum numbers: tell us the properties of atomic
orbitals and the properties of electrons in the orbitals
• Principle quantum number
– Symbol: n
– The energy level that an electron occupies
• Angular momentum quantum number
– Symbol: l
– The shape of the orbital
• Spin quantum number
– +1/2 or -1/2
– Spin state of an electron