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									Modern Atomic Theory
Quantum Numbers
Quantum Numbers
Quantum Numbers
 The Aufbau principle states that energy
 levels must be filled from the lowest to the
 highest and you may not move on to the
 next level unless the previous level is
 full. Use the periodic table as a guide
 Hund’s Rule says that when placing
  electrons in orbitals of equal energy, place
  one in each orbital before doubling up in
  order to arrive at the lowest energy
 The Pauli Exclusion Principle states that
  when electrons do share an orbital, they
  must be of different “spin.”
 Exceptions to the rules
 There are a few exceptions to the rules
 listed above when filling electron
 configurations. A half-full “s” orbital and a
 “d” subshell with 5 or 10 is more stable
 than following the Aufbau Principle. Cr,
 Mo, W: s1 d5 and Cu, Ag, Au: s1 d10
      Electron Configurations
 Orbital information is often described in a
 shorthand notation called an electron

     Electron Configurations

Represents the                 Shows the number of
principal energy level         electrons in the orbital


                                Indicates the shape of
                                the orbital
            Orbital Diagrams
 This information is also described using
 orbital diagrams. Arrows are added to an
 orbital diagram to show the distribution of
 electrons in the possible orbitals and the
 relative spin of each electron

     __      __       __ __ __
      1s        2s             2p
 Writing Electron Configurations
 Determine the number of electrons in the atom
  from its atomic number
 Add electrons to the sublevels in correct order of
 s = 2, p = 6, d = 10, f =14
 To check your complete electron configuration,
  look to see whether the location of the last
  electron added corresponds to the element’s
  position on the periodic table
     Drawing Orbital Diagrams
 Draw a line for each orbital of each sublevel
  mentioned in the complete electron
 Draw one line for each s sublevel, three for each
  p sublevel, 5 for each d sublevel and 7 for each f
 Label each sublevel
 For orbitals containing two electrons, draw one
  arrow up and one arrow down to indicate the
  electrons’ opposite spins.

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