Quantum Mechanics Overview
Quantum: (Latin, "how much") refers to discrete
units (packets/bundles/particles/chunks) that this
theory assigns to certain physical quantities (for
example energy)
Mechanics: branch of physics that deals with
atomic and subatomic systems based upon the
discovery that waves could be measured in particlelike small packets of energy called quanta
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Current Atomic Model: Early
1900’s
Rutherford Model of the atom: concluded through their
experimentation that the atom had a nucleus with an
overall positive charge and negative electrons orbiting the
center
•this model did a poor job explaining why negative
electrons didn’t attract to the positive nucleus
•this model did a poor job how the electrons were
arranged at all
• this model didn’t account for the different behavior of
apparently similar chemicals
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Light: Behave like a Wave
What is a wave? a disturbance that propagates
through space, often transferring energy.
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Light: Behave like a Wave
Classes of waves:
mechanical waves: exist in a medium (travel
through stuff)
electromagnetic radiation waves: can travel
through vacuum (without a medium)
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Light: Wave Properties Describing Waves
frequency (ν): number of waves that pass through
a point per second (unit: 1/s or Hertz (Hz))
wavelength(λ): distance between peaks or troughs
(unit meter with appropriate prefix)
amplitude (A): height of the wave (unit meter
with appropriate prefix)
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Light: Wave Properties
Electromagnetic Radiation: energy that can travel
through empty space in a wavelike fashion at the
same speed (c =3.00 x 108 m/s)
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Light: Wave Properties
Electromagnetic Radiation: the relationship
between frequency (ν) and wavelength(λ) is inverse
and the multiplication equals c (3.00 x 108 m/s).
c= λ ν
With higher
frequency
comes shorter
wavelength and
vice versa.
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Light: Wave Properties
Interference Pattern: look that the following
experiment and predict what will happen
Light behaves as a wave with a characteristic
frequency (ν), wavelength(λ), and amplitude (A).
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Light: Wave Properties
Interference Pattern:
Constructive
the waves add
Destructive
the waves cancel
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Light: Behave like a Particle?
Max Planck: wanted to see why light of only certain
colors was emitted from excited gases or heated
metals
•He looked at emission spectra examples
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Light: Behave like a Particle?
Particles of light are called photons. Effect of photons on
electrons.
Planck found Equantum=hν where E is energy, h is
Planck’s Constant (6.626 x 10-34 J.s and J is Joule the
SI unit of energy) and ν is frequency
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Light: Behave like a Particle?
Photoelectric Effect: electrons are emitted from
the surface of a metal only when light of a specific
frequency is shone
•Wave model of light predicts that with enough
low frequency light energy could build up and emit
electrons from the surface of a metal
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Light: Behave like a Particle?
Photoelectric Effect: electrons are emitted from
the surface of a metal only when light of a specific
frequency is shone
•However, this was not the case because all metals
seemed to have a threshold frequency below which
photoelectric electron ejection did not occur
•This led Einstein to expand on Planck’s ideas and
state Ephoton=hν where E is energy, h is Planck’s
Constant (6.626 x 10-34 J.s and J is Joule the SI
unit of energy) and ν is frequency.
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Current Atomic Model: Early
1900’s
Bohr Model of the atom: agreed that the atom had a
positive nucleus but using the recently discovered ideas of
Planck & Einstein, stated that electrons orbited the
nucleus at certain specific distances and subsequently
energy levels (n)
• this model did a good explaining why negative electrons
didn’t collapse into the positive nucleus
•this model only predicted correct results for hydrogen’s
atomic spectra
•this model did make the false assumption that electrons
orbit the nucleus like planets around the sun
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Electrons: Wave Properties?
Interference Pattern: look that the following
experiment and predict what will happen
• much to the surprise of experimenters electrons
exhibited the same interference patterns in a two
slit experiment similar to that with light
•reinforced the idea that light had particle like
properties (the quantum)
•this led to the conclusion that electrons had wave
like properties
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Quantum Concept
De Broglie Model of the atom: agreed with Bohr but
stated electrons behave as waves as described by the De
Broglie Equation
λ =h/mv where λ is wavelength, h is Plank’s Constant, m
is mass, and v is velocity.
• this early quantum mechanical model did a good
explaining why negative electrons didn’t collapse into the
positive nucleus
• it also explained why different elements only emitted
specific colors of light when excited
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Quantum Concept
Consequence of the Quantum Concept:
Heisenberg Uncertainty Principle: it is not possible
to know both the position and velocity of
electrons because light must be shone on electrons
to determine their position which will then change
their velocity
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Quantum Concept
Schrodinger: agreed with Bohr but considering
the Heisenberg Uncertainty Principle calculated the
probability of finding an electron (as a wave/
particle) in a given 3D space
•the 3D space is called an atomic orbital and
represents a 90% probability of finding an electron
at a given energy level
•worked for all elements
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Quantum Concept
Schrodinger’s Atomic Orbitals: calculated that
different energy levels (n) had different shapes and
sizes
• n (the principal quantum number) can range 1-7
• as n increases the number of possible different
shapes (which are called sublevels) increases
• each sublevel (s, p, d or f) has a certain number of
atomic orbitals (s=1, p=3,d=5,f=7)
• each atomic orbital has a capacity of two
electrons
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