wave nature of lightlms-media.uttyler.edu/fileman/rmason/mason 1311 chap 6...max plank received the...
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• Wave Nature of Light
• Quantized Energy and Photons
• Bohr Model of the Atom
• Wave Behavior of Matter
• Quantum Mechanics / Atomic Orbitals
• Shapes of Orbitals
• Multi-electron Atoms
• Electron configurations and the periodic table
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• To understand the electronic structure of atoms, one must understand the nature of electromagnetic radiation.
• The height of the wave is amplitude (A).
• The distance between corresponding points on adjacent waves is the wavelength (, “lambda”),typically in units of meters.
Section 6
.1
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• The number of waves passing a given point per
unit of time is the frequency (, “nu”) in units of s–1.
• For waves traveling at the same velocity, the
longer the wavelength, the smaller the frequency
(and vice versa).
Higher Frequency
Lower Frequency
Section 6
.1
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• All electromagnetic radiation travels at the same
velocity: the speed of light (c), 3.00 108 m/s.
• Therefore, c = or = c/ or = c/
Section 6
.1
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• The wave nature of light
does not explain how an
object can glow when its
temperature increases.
• Max Planck explained it
by assuming that energy
comes in packets of
matter called quanta.
Section 6
.2
Max Plank received the Nobel Prize in Physics for the quantum
theory in 1918.
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• Planks proposed a
quantum of energy was
related to frequency:
Section 6
.2
E = h h=Plank’s constant
= 6.626x10-34 Js
= frequency (s-1)
• The idea is that energy
can be emitted or
absorbed only in integer
multiples of the frequency
(h, 2h, 3h, etc)
Continuous energy
Quantized energy
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• Einstein used this assumption to explain the photoelectric effect.
• He showed photon’s energy is proportional to its frequency:
E = h
h = Planck’s constant = 6.63 10−34 J·s
(energy is in J)
Albert Einstein received the Nobel Prize in Physics
for the photoelectric effect, not for relativity (E=mc2).
Section 6
.2
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So, if the wavelength of light is
known, one can calculate the
energy in one photon, or
quanta, of that light:
c = and E = h
E = hc/
Q: But photons have mass, so is
light a particle or is it a wave?
A: Yes!
This paradox is called wave-particle duality.
Section 6
.2
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Researchers at Ecole
Polytechnique Federale
de Lausanne (EPFL)
have captured light
behaving as a wave and
a particle at the same
time!
http://actu.epfl.ch/news/t
he-first-ever-photograph-
of-light-as-both-a-parti/
https://www.youtube.com/watch?t=46&v=mlaVHxUSiNk
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– Calculate the frequency of laser radiation which
has a wavelength (l) of 640.0 nm.
– Calculate the wavelength of radio waves with a
frequency of 103.4 MHz (1 MHz = 1 × 106 s–1).
Section 6
.2
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– What is the energy of one photon of radiation
with a frequency of 4.69 × 1014 s–1?
– What is the total energy of a pulse of 5.0 × 1017
photons at this frequency?
Section 6
.2
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• If a laser emits 1.3 × 10–2 J of energy, how many
photons were emitted?
Section 6
.2
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Another mystery involved the emission spectra
observed from energy emitted by atoms and
molecules.
Section 6
.3
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• White light sources produce
a continuous spectrum.
• Atoms and molecules only
produce a line spectrum of
discrete wavelengths.
Section 6
.3
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Niels Bohr’s explanation:
1. Electrons in an atom can only orbit at certain radii, corresponding to specific energies.
Section 6
.3
E = h h = Planck’s constant
= frequency of radiation
2. Electrons in permitted orbits have specific, “allowed” energies; these energies will not be radiated from the atom.
3. Energy is only absorbed or emitted as an electron moves from one “allowed” energy state to another; the energy is defined by:
n=1
n=2
n=3
n=4
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Energy that’s absorbed (promotion) or emitted (demotion) can be calculated by the Rydberg equation:
RH = Rydberg constant, 1.097 107 m–1
h = Planck’s constant; c = speed of light
ni = initial energy level of electron
nf = final energy level of the electron
22
11
if
Hnn
hcRE
22
18 111018.2
if nnxE
Section 6
.3
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+ -
• Moving from high n to lower n
– nf< ni so E<0
– Emits energy
• Moving from low n to higher n
– nf > ni so E > 0
– Absorbs energy
22
11
if
Hnn
hcRE
Section 6
.3
Ground State = lowest energy state
Excited State = higher energy state
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• Strengths of the Bohr model:
– Electrons have discrete energies
– Energy is absorbed or radiated when electrons are moved between discrete levels
• Limitations of the model
– Designed to work for hydrogen atom only. Other elements/substances are much more complicated
– Electrons are not merely particles circling the nucleus of atoms, they exhibit wave-like properties
Section 6
.3
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• Do the following transitions represent absorption of energy or emission of energy:– n = 3 to n = 1?
– n = 2 to n = 4?
Section 6
.4
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• DeBroglie Equation: Relationship
between mass and wavelength:
=h
mv
= wavelength (meters)
h = Planck’s constant (6.63 × 10–34 J·s)
m = mass (kg)
v = velocity (meters/s) Reminder: 1 J = 1 kg·m2/s2
• Louis de Broglie suggested that if light
(photons) has material properties, then
matter should exhibit wave properties.
Section 6
.4
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• Calculate the velocity of a neutron whose de Broglie wavelength is 500 pm.
(Neutron mass = 1.675 x 10–24 g)
Section 6
.4
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• The more precisely the momentum (mv) of a particle
is known, the less precisely its position (x) is known
(and vice versa):
• The uncertainty of an electron’s position can be greater
than the size of the atom! (see pg 225!)
(x)(mv) h
4
Section 6
.4
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• Use a camera to photograph a car in motion.
• Short exposure shot: good idea of the car’s position, but not of it’s speed
• Long exposure shot: good idea of speed, but position is blurry
Section 6
.4
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• Doesn’t apply to large objects (e.g. a
tennis balls or people) because
(x)(mv) is much smaller than the
size of the object.
m
smkg
Js
vm
hx 34
34
1006.4/894.0145.04
10626.6
4
m
smkg
Js
vm
hx 9
431
34
101/10510109.9
10626.6
4
Baseball (d=0.075m) at 90 mph with an uncertainty of 2 mph:
Electron at (d=<10-10m) at a bit over 1million mph with uncertainty of a bit over 100,000 mph:
Section 6
.4
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• Quantum mechanics -
mathematical treatment into which
both the wave and particle nature
of matter is incorporated
• Developed by Erwin Schrödinger
As we delve deeper into the atom, it actually becomes
less tangible and more mathematical.
Section 6
.5
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• The “wave” equation is
designated with a lower
case Greek psi ().
• The square of this
equation,2, gives a
probability density map.
Section 6
.5
– More blue dots = more likely to be there
– This tells us where an
electron is statistically likely
to be at any given instant.
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• Solving the wave equation gives a set of wave functions, or orbitals, and their corresponding energies.
• Orbitals describe a spatial distribution of electron density (90% probability of finding a particular electron or electrons).
• Quantum numbers are used to describe each orbital. A total of three quantum numbers is required to describe a specific orbital.
Section 6
.5
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• The principal quantum number, n, describes the energy level on which the orbital resides.
• The values of n are whole numbers > 0.
– e.g. n = 1, n = 2, n = 3, etc
– Same n as in the Bohr model
– n is equal to the number of the row on the periodic table
Section 6
.5
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• The azimuthal quantum number, l (“ell”), defines the
shape of the orbital.
• Possible values for l: integers ranging from 0 to n−1.
• Letter designations of l are used to describe the
different values (i.e. shapes) of orbitals.
Value of l 0 1 2 3
Type of orbital s p d f
Section 6
.5
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• The magnetic quantum number, ml, describes the three-dimensional orientation of the orbital.
• Possible values are integers ranging from +l to –l(including zero):
-l ≤ ml ≤ +l
Example: l = 1 ml = -1, 0, +1 (3 values)
• Therefore, on any given energy level (n), there can be up to: – 1 s orbital, 3 p orbitals, 5 d orbitals, 7 f orbitals, etc.
which of these are available depends on n and then l.
Section 6
.5
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• All the orbitals with the same value of n form a shell.
• Different orbital types within a shell are subshells.
Section 6
.5
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• A shell with principal
quantum number n has
exactly n subshells
– n=2 2 subshells (l=0, l=1)
• Each subshell has 2l+1
orbitals – l=0 ml=0
– l=1 ml=1,0,-1
• The total number of
orbitals in a shell is n2.– n=2 4 (count ml values)
Section 6
.5
Each box represents
1 orbital
Each group represents 1 subshell
Each row represents
1 shell
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• Predict the number of subshells in the fourth shell (n=4).
• Give the label for each of these subshells.
• How many orbitals are in each of these subshells?
Section 6
.5
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• Value of l = 0.
• Spherical in shape.
• Radius of the sphere
increases as n increases.
– rn=1<rn=2<rn=3< …
Section 6
.6
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The s orbitals possess n−1 nodes, or regions where
there is zero probability of finding an electron.
Section 6
.6
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• Value of l = 1.
• Two lobes with 1 node between them.
• ml = –1, 0, +1 and thus 3 different spatial orientations
For convenience we place the lobes along the x, y and z axes.
Section 6
.6
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• Value of l is 2; five values of ml (+2, +1, 0, –1, –2)
• Four of the five orbitals have 4 lobes; the other
resembles a p orbital with a ring around the center.
Section 6
.6
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• For a one-electron
hydrogen atom, orbitals
on the same energy level
have the same energy
(degenerate).
• But this is only theory,
what we observe is…
Section 6
.7
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• As the number of
electrons increases,
so does the repulsion
between them.
• In many-electron
atoms, orbitals in the
same energy level are
no longer degenerate.
Specific orbitals are described by n, l and ml, but 2 electrons
can be located in an orbital. How can we distinguish them?
Orbitals in the same subshell
are degenerate
Section 6
.7
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• In the 1920s, it was found that two electrons in the
same orbital do not have exactly the same energy.
• The “spin” of an electron describes its magnetic field,
which affects its energy.
Section 6
.7
• This led to a 4th quantum
number, the spin quantum
number, ms.
• The spin quantum number
has only 2 allowed values:– ms = +½ or ms = −½.
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• No two electrons in the same atom can have exactly the same energy.
• No two electrons in the same atom can have four identical quantum numbers.
• Each orbital can only hold two electrons (and they must have opposite spins, ms = +½, –½).
Section 6
.7
• Each electron’s “address” is a unique combination of the 4 quantum numbers
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No two orbitals can have the exact same n, l, and ml values!
No two electrons can have the exact same n, l, ml, and ms
values!
Symbol Name Description Values Misc. notes
n principal Energy level of orbital
Integers > 0 n corresponds to a row on periodic table
l azimuthal(angular)
Shape of orbital Integers 0 to n–1
0 = s 1 = p2 = d 3 = f
ml Magnetic 3D orientation of orbital
Integers–l to +l
Gives number of orbitals for different
values of l
ms Spin Electron spin +½, –½ Only describes electrons, not orbitals
Section 6
.7
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• Consists of
– Number denoting the energy
level. Corresponds to principle QN, n
– Letter denoting the type of
orbital. Corresponds to azimuthal QN, l
– Superscript denoting the
number of electrons in those
orbitals.
4p5
Section 6
.8
• Tally of where each electron is in an atom.
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• What is the designation for the subshell with n=5 and l=1?
• How many orbitals are in this subshell?
• Indicate the values for ml for each of these orbitals.
Section 6
.7
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• Write the electron configuration for Li atom.
Section 6
.7
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• Each box represents one orbital.
• Half-arrows represent the electrons.
• The direction of the arrow represents the spin of
the electron.
• Orbital diagrams represent the “ground state” or
most stable electron configuration
Section 6
.8
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“For degenerate orbitals, the lowest energy is
attained when the number of electrons with the
same spin (ms) is maximized.”
Don’t pair up electrons until after you’ve half-filled a subshell.
Nitrogen has 7 electrons:
Section 6
.8
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• Draw the orbital diagram for the electron configuration of oxygen (atomic number 8).
• How many unpaired electrons does an oxygen atom have?
Section 6
.7
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• Write the full electron configuration for P, element 15.
• How many unpaired electrons does a P atom possess?
Section 6
.8
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• Fill orbitals in increasing order of energy
• Rows correspond to principal quantum numbers
• Groups correspond to different electron configurations.
s1 s2
s2… p1 p2 p3 p4 p5 p6
d1 d2 d3….s2…n = 1
n = 2n = 3n = 4n = 5n = 6n = 7
Section 6
.8
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1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f 5g
6s 6p 6d 6f 6g 6h
7s 7p 7d 7f 7g 7h 7i
8s
Same order as previous slide
Elements with orbitals in lighter
shade aren’t known (yet!)
Filling from lowest to
highest energy level
is called the Aufbau
Principle
Section 6
.8
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• Not sure which orbitals are lowest energy? – Fill lowest sum of n+l first
– Ties go to lower n
• What order should we fill
– Check n+l what order should we fill 3d, 4s & 4p?
3d: n=3, l=2 n+l =5
4s: n=4, l=0 n+l =4
4p: n=4, l=1 n+l =5
• Fill order: 4s, 3d, 4p
Section 6
.8
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• Group 8A elements are said to have “filled shells”.
• Filled shell electrons are called “core electrons” and do
not participate in making bonds or redox reactions.
• Electrons in the “outer shell” are “valence electrons” and
do participate in bonding and redox reactions.
• We write condensed electron configurations by writing
the next lowest noble gas configuration (the core
electrons) plus the valence electrons
• Examples: P – 1s22s22p63s23p3 [Ne]3s23p3
Ge – 1s22s22p63s23p34s23d104p2
[Ar]4s23d104p2
Section 6
.8
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• Write the condensed electron configuration for Te (element 52).
Section 6
.9
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• Which element has an electron configuration of [Kr]5s24d3 ?
• How many unpaired electrons are there in this element?
Section 6
.9
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• For predicting reactivity, the
valence shell configuration is
most important.
Section 6
.9
S
– We don’t usually consider
electrons in completely filled d
or f subshells
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• What family of elements is characterized by
an ns2np2 electron configuration in the
outermost occupied shell?
• What family of elements is characterized by
an ns2np5 electron configuration in the
outermost occupied shell?
Section 6
.9
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• 4s and 3d orbitals are very close in energy
• Energetically favorable to half-fill or fully fill the d orbitals
Section 6
.9
• Similar logic also applies to the f-block
Order of orbital stability 1. Full or empty subshell 2. Half –filled 3. Everything else
• If the energy
cost very low,
re-arrange to
make more
stable
subshells
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Section 6
.9
Chromium: [Ar] 4s13d5
not [Ar] 4s23d4
Copper: [Ar] 4s13d10
not [Ar] 4s23d9
4s 3d
Full + Random
½ Full + Full
More Stable
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• When writing electron configurations remember: 1. Fill lowest energy levels first (Aufbau Principle)
Lowest sum of n+l goes first
Ties go to the lowest n
2. Limited to 2 electrons per orbital so s subshell gets 2 max
p subshell gets 6 max
d subshell gets 10 max
f subshell get 14 max
3. Be aware of anomalies Above 4s, bump an electron up to the d or f
subshell if doing so will create a combination of full,
empty or half-filled shells.
Chapte
r 6
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• Calculations involving E, l, n, m and v
• Bohr hydrogen atom model
• Electron transitions and Rydberg equation
• Energy states
• Strengths and limitations of the model
• DeBroglie equation, uncertainty principle (define
and calculate)
• What are orbitals and probability densities?
Chapte
r 6
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• Quantum numbers, orbital shapes, shells,
subshells, and nodes
• Ordering of orbitals: degenerate vs non-
degenerate, Pauli exclusion principle
• Writing electron configurations, Hund’s rule,
condensed electron configurations, electron
configurations of groups on periodic table
Chapte
r 6