Ionic Bonding

1
Chapter 5

• Ionic Bonding

2
Ionic Bonding

• Transfer of electrons and the attraction between
  the resulting charged ions

1903 Nobel Prize in Chemistry
Svante Arrhenius (1859-1927)
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Characteristics of Ionic Compounds

• All ionic compounds are solids
• crystals are hard and brittle
• high melting points
• when heated to the molten state, they conduct
  electricity
• dissolve in high-polarity solvents, solutions
  become electrically conducting

4
The Ionic Model

• Ionic bond
• electrostatic attraction between a cation and an
  anion
• large electronegativity difference between the
  two ions
• hardly any bonds are truly covalent or ionic

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The Ionic Model

• Typically between metals and nonmetals
• electrons are transferred from the element of
  lower electronegativity to the element of higher
  electronegativity

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Size of Ions

• Cations are smaller than the neutral atoms

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Size of Ions

• Anions are larger than the neutral atoms

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Trends in Ionic Radii

• Generally, metal cations are smaller than the
  nonmetal anions
• Down a group, ions become larger as the atom
  becomes larger

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Trends in Melting Points

• Must overcome the ionic attractions to allow ions
  to move freely
• The smaller the ion, the higher the melting point
• The higher the charge, the higher the melting
  point

10
Polarization

• Anion valence electrons significantly attracted
  to the cation, giving covalent character
• causes distortion
• Kasimir Fajans gave three rules for polarization
• a cation will be more polarizing if it is small
  and highly positively charged
• an anion will be more polarizable if it is large
  and highly negatively charged
• polarization is favored by cations that do not
  have a noble-gas configuration

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Charge Density

• a measure of the polarizing power of a cation
• for Na,
• (ion charge 1, ionic radius 116 pm)
• for Al3,
• charge density 364 C/mm3

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Ionic vs. Covalent

• Observe melting points
• AlF3 (m.p. 1290C) and AlI3 (m.p. 190C)
• F- (117 pm) and I- (206 pm)
• Ionic charge
• 1 and 2, ionic predominates
• 3, covalent predominates
• 4 and above, only covalent
• charge density is so high that it polarizes all
  anions

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Ionic vs. Covalent

• Ions which do not have a noble-gas configuration
• such as Ag, Cu, Sn2, and Pb2
• NaCl (m.p. 801C) and KCl (m.p. 770C)
• AgCl (m.p. 455C)
• NaCl and KCl highly soluble in polar solvents
• AgCl is highly insoluble

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Hydration of Ions

• formation of ion-dipole interactions allows
  dissolution of ionic compounds in water

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Hydrates

• When ionic compounds are crystallized from an
  aqueous solution, water is often incorporated in
  the structure
• typically associated with the cation
• AlCl36H2O
• water molecules are arranged in an octahedral
  arrangement surrounding the aluminum
• Al(OH2)633Cl-

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Hydration

• Extent of hydration depends upon charge density
• low charge densities do not form hydrates
• NaCl and KCl
• 3 ions almost always results in a hexahydrated
  ion
• highly charged oxyanions are almost always
  hydrated
• ZnSO47H2O
• Zn(OH2)62SO4(H2O)2-

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The Ionic Lattice

• Form the same basic crystal lattices as discussed
  in chapter 4
• anions make up the lattice
• cations fill the holes (or interstices)
• Three general principles apply to ionic lattices

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Ionic Lattice Principles

• Ions are assumed to be charged, incompressible,
  nonpolarizable spheres.
• Ions try to surround themselves with as many ions
  of the opposite charge as possible and as closely
  as possible.
• The cation to anion ratio must reflect the
  chemical composition of the compound.

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Radius Ratio

• Ratio of the cation radius to the anion radius
• denoted as r/r-

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Cubic Unit Cell

• space-filling vs. ionic lattice diagrams
• diagonal has a length of the sum of two anion
  radii and two cation radii
• 11 ratio

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Cubic Unit Cell

• 21 Ratio
• Seen in the fluorite structure (CaCl2)

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Octahedral Holes
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Tetrahedral Holes
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Octahedral Unit Cell

• 6 atoms surround the cation
• different structures have octahedral holes
• fcc
• bcc

TiO2
NaCl
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12
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Tetrahedral Unit Cell

• cations fill tetrahedral holes
• hcp
• ccp

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Exceptions to Packing Rules

• Many exceptions exist for the radius ratio
  criterion
• due to partial covalent bonding, and actual
  non-hard spheres

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The Bonding Triangle