10/28/09
Electron Configurations
Describing where the unknown
locations of electrons are
Quantum Numbers
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The four quantum numbers are:
Principle, n
Angular momentum, l
Magnetic, ml
Electron spin, ms
We haven’t said much about electron
spin, have we?
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Electron Spin
•  The first three quantum numbers were
solutions to the Schrödinger equation
•  The presence of electron spin effects
were determined experimentally
•  Do electrons really “spin?”
•  Not in a classical sense, but the effects
observed are the same as for spinning
magnets
Electron Spin
•  For lack of a better term, this property
was called spin
•  What are the possible values for ms?
•  +½ and –½
Electron Filling Series
•  In an atom, which electronic orbitals fill
first?
•  A few rules help us
•  Aufbau Principle
•  Pauli Exclusion Principle
•  Hund’s Rule
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Electron Filling Series
•  Aufbau Principle
–  Electrons will occupy lowest energy
orbitals first
•  Pauli Exclusion Principle
–  No two electrons in an atom have the
same set of quantum numbers
Electron Filling Series
•  How do we know which orbitals have
lowest energy?
•  Diagonal method
•  Periodic Table
•  How many electrons fit in each
orbital?
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Figure 8.12 p. 307
Examples
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Electron configuration for Si
How many electrons?
14
1s22s22p63s23p2
Electron configuration for Co
27 electrons
1s22s22p63s23p64s23d7
Orbital Diagrams
•  Show electron spin as well as
numbers of electrons
•  N—7 electrons
•  ↑_ __
__ __ __
1s
2s
2p
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Orbital Diagrams
•  Show electron spin as well as
numbers of electrons
•  N—7 electrons
•  ↑↓ __
__ __ __
1s
2s
2p
Orbital Diagrams
•  Show electron spin as well as
numbers of electrons
•  N—7 electrons
•  ↑↓ ↑_
__ __ __
1s
2s
2p
Orbital Diagrams
•  Show electron spin as well as
numbers of electrons
•  N—7 electrons
•  ↑↓ ↑↓
__ __ __
1s
2s
2p
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Orbital Diagrams
•  Show electron spin as well as
numbers of electrons
•  N—7 electrons
•  ↑↓ ↑↓
↑_ __ __
1s
2s
2p
Orbital Diagrams
•  Show electron spin as well as
numbers of electrons
•  N—7 electrons
•  ↑↓ ↑↓
↑_ __ __
1s
2s
2p
•  Hund’s Rule
Orbital Diagrams
•  Show electron spin as well as
numbers of electrons
•  N—7 electrons
•  ↑↓ ↑↓
↑_ __ __
1s
2s
2p
•  Hund’s Rule
•  The lowest energy has the most
parallel spins
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Orbital Diagrams
•  Show electron spin as well as
numbers of electrons
•  N—7 electrons
•  ↑↓ ↑↓ ↑ ↑ ↑_
1s 2s
2p
•  Hund’s Rule
•  The lowest energy has the most
parallel spins
Continuing example
•  Orbital diagram for Si
•  1s22s22p63s23p2
•  ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑ ↑ __
1s 2s
2p
3s
3p
Electron Configuration Video
•  Electron-Electron repulsion affects the
energy of some subshells.
•  This is why the 3d comes after the 4s.
•  There are some other exceptions, but we
will cover these tomorrow.
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Examples
•  He
•  P
•  Ge
Electron Shielding
•  As e- are positioned further and further
from the nucleus their energy increases.
•  However, electron repulsion causes a
reduction of some energy levels
So what is the deal with Sc?
•  1s22s22p63s23p64s23d1
•  The energy in the 4s and 3d energy levels
are very close to one another, but the 4s
just a little smaller.
•  Now look at Cr: [Ar] 4s13d5
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Stability in shells
•  Half filled d shells are more stable than a
full s and a partially filled d.
•  This mainly occurs in the transition metals
found in the middle of the periodic table.
Valence Electrons
•  e- that participate in chemical reactions.
•  Octet rule; atoms want to have eight
valence electrons.
•  Or….fill or empty shells easily.
•  Deals primarily with s and p orbitals.
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