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Chemistry Notes
The Quantum Theory
The Bohr Model
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Recall: Bohr’s model describes the atom as having
definite energy levels
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e- will absorb a “quantized” amount of energy, which
excites the e-,when e- returns to its ground state it
always emits light at specific wavelengths
These wavelengths correspond to definite changes in
the electrons energy
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Bohr used Max Plank’s idea: E = hc/λ
Bohr’s model refers to quanta as packets of
energy (or particles)
From Bohr to the Quantum Model
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Bohr’s model had issues
considers electrons to have both a known
radius and orbit
 It makes poor predictions regarding the
spectra of larger atoms.
 It does not predict the relative intensities of
spectral lines.
 The Bohr Model does not explain fine
structure in spectral lines.
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From Bohr to the Quantum Model
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Heisenberg Uncertainty Principle: it is
impossible to know the exact position and the
exact momentum of an electron at the same
time. (This challenged Bohr’s Model)
In order to know where an
electron is we have to “see”
it. To see something it must
be hit by a photon. Photons
and electrons are roughly the
same size, when they collide
the electron will no longer be
in the same position.
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From Bohr to the Quantum Model
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Photoelectric Effect showed that r
frequency high light can eject electrons.
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i.e. – Light can act as a particle  See
Physics notes
De Broglie – can quanta (particles) have
wave properties? Can particles act like
waves?
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Wave-particle duality
From Bohr to the Quantum Model
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Erwin Schrodinger investigated the wave
nature of the electron

Developed a mathematical equation that
related the electron's amplitude to any point in
space around the nucleus
Schrodinger's Equation
• Ψ is the wave function or probable position of
the electron.
•m is the mass of the electron.
•E is the total energy of the system.
•V is the potential energy and is a function of x, y
and z.
The probability of finding the electron decreases
as you move away from the center of the nucleus.
Schrodinger's Equation

The solutions of the Schrodinger equation
can tell us things about the electron.
The size of electron cloud
 The energy of the electron cloud
 The shape of the cloud

The Quantum Model

Summary: Quantum model seeks to
describe the behavior of subatomic
particles in terms of waves

Electrons have properties of both waves
and particles
The Quantum Model
Bohr’s model showed definite positioning
of electrons (they were in orbits).
 The Quantum model is primarily a
mathematical one that shows the probable
location of an electron

Notice: the circles
denote the probability
of finding an electron in
that region of space.
The Quantum Model

A fuzzy cloud represents
the probability of finding
an electron within a
certain volume of space.
This Fuzzy Cloud is called
an orbital
Denser cloud = higher
probability of finding an
electron
Less dense cloud = lower
probability of finding an
electron
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The Quantum Model
Orbits (Bohr’s Model) vs. Orbitals (Quantum Model)
(PATHS e- travel in
(Regions of space where e- may be)
The Quantum Model

Letters that denote atomic orbitals in
sublevels:
s – spherical shaped
 p – dumbbell shaped (3 orbitals)
 d – four of the five kinds of d orbitals are
clover shaped. (5 orbitals)
 f – very oddly shaped (7 orbitals)
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The maximum number of electrons in each
orbital is 2.
AGAIN!!!
The Quantum Theory is not trying to show
what the atom looks like.
 The QT is a mathematical model

The QT is trying to show us the probable
location of electrons in an atom
 The orbitals represent the probability of
finding an electron

Describing the Distribution of
Electrons
Quantum Numbers: like a zip code for electrons
n (principle quantum number)– Indicates
principle energy level

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Describes relative size of the electron cloud
Corresponds to the energy level 1,2,3,…n
Describing the Distribution of
Electrons
l (second quantum number) – Indicates
sublevel
 Describes shape of cloud
 s: l = 0; p: l = 1; d: l = 2; f: l = 3
Describing the Distribution of
Electrons
m (third quantum number) – Indicates
orientation in space of the cloud
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For the p orbitals the values range from –1 to 1
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For the d orbitals the values range from –2 to 2
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px (-1) / py (O) / pz (+1)
dx2y2 (+2) / dz2 (+1) / dxy (O) / dxz (-1) / dyz (-2)
For the f orbitals the values range from –3 to 3
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Don’t worry about it.
Describing the Distribution of
Electrons
s (fourth quantum number) - distinguish
between electrons in same orbital.
 +1/2 denotes a clockwise spin
 -1/2 denotes a counter clockwise spin.
 The
Pauli exclusion principal says: electrons in
the same orbital have opposite spins. (See Fig.
5.12 on p. 123 in book)
Describing the Distribution of
Electrons
What do the following numbers tell us: 4, 1, -1, +1/2
Electron is in the 4th PEL
 Electron is in the p sublevel
 Electron is oriented in the orbital along xaxis
 Electron has a clockwise spin

Describing the Distribution of
Electrons
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Quantum numbers are like zip codes
N  United States / PEL
 L  Michigan / sublevel
 M  Bridgman / orbital
 S  facing north / spin

See table 5.1 in book
 Please read pages 109-125

The Quantum Theory
Summary
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Explain how the quantum model is part of the on
going development of the atomic model.
State the Heisenburg Uncertainty Principle and
explain how it influenced the modern view of the
atom.
Explain how the Quantum Model is different than
other views of the atom.
Use quantum numbers to locate the
approximate location of electrons.