Chapter 8: Electron configurations and periodicity

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Chapter 8 Electron configurations and periodicity
Chemistry 1061 Principles of Chemistry I Andy
Aspaas, Instructor
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Electron spin and configurations

• Electrons may have only one of two possible spins
• Quantum number ms 1/2 or -1/2
• Similar to poles of a magnet
• Electron configuration distribution of electrons
  among available subshells
• Notation lists one subshell after another, with
  number of electrons in each given in superscript
• Ex. 1s22s1 means 2 electrons in 1s and 1 electron
  in 2s
• Orbital diagram drawing of orbitals and their
  occupations
• Electrons indicated by arrows, either up or down
  indicates spin

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Orbital occupancy

• Pauli exclusion principle no 2 electrons in the
  same atom may have the same 4 quantum numbers
• Each orbital may contain two electrons, but they
  must be opposite spin - same-spin pair in an
  orbital is forbidden
• Each subshell can hold twice as many electrons as
  orbitals in the subshell

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Subshell occupancies
Subshell Number or orbitals Maximum number of electrons
s (l 0) 1 2
p (l 1) 3 6
d (l 2) 5 10
f (l 3) 7 14
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Filling orbitals

• Every atom has an infinite number of electron
  configrations, but the ground state (stablest)
  electron configuration can be predicted
• Aufbau principle electron configurations can be
  predicted by successively filling subshells with
  electrons in a specific order
• electrons in a neutral atom atomic number

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Using the periodic table to fill orbitals

• Use the row number to determine n for s and p
  sublevels
• Section of the periodic table corresponds with
  the subshell thats added
• First two groups s
• Last 6 groups p
• Transition metals d
• Inner transition metals f

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Available subshells

• 1s
• 2s, 2p
• 3s, 3p, 3d
• 4s, 4p, 4d, 4f
• 5s, 5p, 5d, 5f
• 6s, 6p, 6d, 6f
• 7s, 7p, 7d, 7f

• These subshells are allowed based on allowed
  quantum numbers
• But, they are filled based on the order they
  appear in the periodic table
• 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 etc

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Writing electron configurations

• Just follow the periodic table from the start,
  adding electrons in the appropriate sublevel
• Electron configurations can be abbreviated using
  a noble-gas core (in brackets)
• Ex. Ca 1s22s22p63s23p64s2 or just Ar4s2

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Electron addition to subshells

• Hunds rule lowest energy arrangement of
  electrons in a subshell is obtained by first
  putting in as many same-spin electrons as
  possible before pairs are made
• Oxygen 1s22s22p4 therefore has 2 unpaired
  electrons in the 2p subshell
• Any atom with unpaired electrons is paramagnetic
  (attracted to a magnetic field due to unpaired
  electrons)