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Sec. 2.1  Atoms and Molecules
1
Structure of atoms
(NE 122 Review)
Bohr
Electrons around a nucleus can be only in discrete
orbits and they absorb or emit energy in discrete
amounts as they move from one orbit to another.
Each
orbit thus corresponds to a definite energy level for the
electron.
Absorption of
electromagnetic
radiation promotes
an electron to higher
energy level
Emits exactly the same
amount of energy when
the electron drop back to
the lower energy level
Calculations could be completed for simple molecules He
+
, Li
2+
, but
could not calculate the energy levels for more complex molecules
because orbitals were assumed to be spherical
.
Vollhardt: Chapter 1
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2
Then came Quantum Mechanics
The basics
1)
Atoms or molecules can only exist in certain energy states.
When an
atom or molecule changes its energy sate, it must emit or absorb just
enough energy to bring it to a new energy state. (the quantum
condition)
2)
The energy change in the atom or molecule is related to the frequency
or wavelength of the light emitted or absorbed by the equation
Δ
E = h
ν
or
Δ
E = hc/
λ
.
Which is equal to the energy of
a photon emitted or
absorbed during the transition.
3)
The allowable states of atoms and molecules can be described by sets
of numbers called
quantum numbers
•
Electrons travel in waves
and can be described by the Schrodinger
equation or a wave function
Ψ
.
•
Quantum numbers
describe the electron configuration
of an atom or
molecule and also describes the shape of the orbitals.
Sec. 2.1  Atoms and Molecules
3
Principle
main
energy
level
Subsidiary
designates
shape or
region of
space
electron
occupies
Magnetic
special
orientation
Spin
Electron
Capacity
of sublevel
Electron Capacity of
Energy Level
n
l
m
l
m
s
= 4l+2
2n
2
1
0 (
1s
)
0
+ ½ ,  ½
2
2
2
0 (
2s
)
0
+ ½ ,  ½
2
8
1 (
2p
)
1, 0, +1
±
½for
each m
l
6
p
x
, p
y
, p
z
18
3
0 (
3s
)
0
±
½
2
1 (
3p
)
1, 0, +1
±
each m
l
6
2 (
3d
)
2, 1, 0, +1, +2
±
each m
l
10
Quantum Numbers
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4
The probability of finding an electron is the square of the wave function (
Ψ
2
).
The electron density is proportional to r
2
Ψ
2
.
r = radius from nucleus
r
2
Ψ
2
(3s)
(1s)
(2s)
An elegant orbital visualization resource, created at MIT can be found at:
http://web.mit.edu/3.091/www/orbs/
Sec. 2.1  Atoms and Molecules
5
+

n = 2
l =1
p
x
p
y
p
z
m
1
= 1, 0 +1
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6
Periodic Table
Build up the periodic table by adding protons, neutrons and electrons to
give the lowest total energy for the atom.
No two electrons can have the same set of quantum numbers
Electronic Configuration
1s
2s
2p
3p
4p
3d
3s
4s
4d
4f
7 obitals
PE
Orbitals that have the same energy
level are called degenerate.
Sec. 2.1  Atoms and Molecules
7
What is the groundstate electronic configuration of Carbon (
6
C)?
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This note was uploaded on 07/18/2011 for the course NE 122 taught by Professor Nano during the Winter '09 term at Waterloo.
 Winter '09
 Nano

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