Quantum Mechanical Model: Electron Configurations

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Quantum Mechanical ModelElectron Configurations

• Chemistry 11

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Electron Configurations

• It is the nature of things to seek the lowest
  possible energy.
• High energy systems are unstable and tend to lose
  energy in order to become more stable.
• In terms of the atom, electrons and the nucleus
  interact to form the most stable arrangement
  possible.

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Electron Configuration

• The ways in which electrons are arranged around
  the nuclei of atoms.

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Rules to Remember

• Three rules govern the filling of atomic orbitals
  by
• electrons within the principle energy levels.
• Aufbau Principle
• Pauli Exclusion Principle
• Hunds Rule

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Aufbau Principle

• Electrons enter orbitals of lowest energy first
• The various orbitals within a sublevel of
  principle energy level are always equal.
• Within a principle energy level, the s orbital
  is always the lowest energy level.
• The range of energy levels within a principle
  energy level can overlap the energy levels of an
  adjacent principle energy level.

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Using the Aufbau Principle
Note 4s orbital is lower in energy than 3d. 4f
orbital is lower in energy than 5d.
As you can see from the diagram the filling order
is as follows 1s 2s 2p 3s 3p 4s 3d 4p 5s
4d 5p 6s
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Energy Level Template
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Pauli Exclusion Principle

• An atomic orbital contains a maximum of 2
  electrons
• In order to occupy the same orbital, these
  electrons must have opposite spins (clockwise and
  counter clockwise)
• Vertical arrows are used to show directions of
  spins
• Paired electrons

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An Easy Way to Remember
1s2 holds 2 electrons
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Fill the Orbitals From the Bottom-up
1s2 2s2 holds 4 electrons
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Fill the Orbitals From the Bottom-up
1s2 2s2 2p6 3s2 holds 12 electrons
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Fill the Orbitals From the Bottom-up
1s2 2s2 2p6 3s2 3p6 4s2 holds 20 electrons
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Fill the Orbitals From the Bottom-up
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 holds 38
electrons
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Fill the Orbitals From the Bottom-up
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6
6s2 holds 56 electronsand so on!
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Hunds Rule

• When electrons occupy orbitals of equal energy,
  one electron enters each orbital until all
  orbitals contain one electron with parallel
  spins.
• Second electrons then add themselves to the
  orbital to pair their spins

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Using Hunds Rule

• When electrons occupy orbitals of equal energy
  they dont pair up until they have to.
• Lets determine the electron configuration for
  Phosphorusenergy level diagrams
• Need to account for 15 electrons

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• The first to electrons go into the 1s orbital
• Notice the opposite spins
• Only 13 more

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• The next electrons go into the 2s orbital
• only 11 more

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• The next electrons go into the 2p orbital
• Only 5 more

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• The next electrons go into the 3s orbital
• Only 3 more

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• The last three electrons go into the 3p orbitals.
• They each go into separate shapes
• 3 unpaired electrons
• 1s22s22p63s23p3

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Remember the following

• Lowest energy to higher energy.
• Adding electrons can change the energy of the
  orbital.
• Half filled orbitals have a lower energy.
• Makes them more stable.
• Changes the filling order