CH 9: Ionic and Covalent Bonding

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CH 9 Ionic and Covalent Bonding

• Renee Y. Becker
• Valencia Community College
• CHM 1045

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Covalent Bonds

• Covalent bonds are formed by sharing at least one
  pair of electrons.
• The attraction (nucleus/electrons) outweighs the
  repulsions (electron/electron nucleus/nucleus)

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Covalent Bonds

• Every covalent bond has a characteristic length
  that leads to maximum stability. bond length

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Strength of Covalent Bonds

• Energy required to break a covalent bond in an
  isolated gaseous molecule is called the bond
  dissociation energy.
• Same amount of energy released when the bond forms

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Example 1

• Which of the following is correct?
• Energy is absorbed to form a bond
• Energy is released when a bond is formed

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Polar Covalent Bonds

• Bond polarity is due to electronegativity
  differences between atoms.
• Pauling Electronegativity is expressed on a
  scale where F 4.0

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Pauling Electronegativities
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Electron-Dot Structures

• Using electron-dot (Lewis) structures, the
  valence electrons in an element are represented
  by dots.
• Lewis symbols
• Valence electrons are those electrons with the
  highest principal quantum number (n).

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Electron-Dot Structures

• The electron-dot structures provide a simple, but
  useful, way of representing chemical reactions.
• Ionic
• Covalent

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Electron-Dot Structures

• Single Bonds
• Double Bonds
• Triple Bonds

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Drawing Lewis-Dot Structures

• Rule 1 Count the total valence electrons.
• Rule 2 Draw the structure using single bonds.
• Rule 3 Distribute the remaining electron pairs
  around the peripheral atoms.
• Rule 4 Put remaining pairs on central atom.
• Rule 5 Share lone pairs between bonded atoms to
  create multiple bonds.

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Drawing Lewis-Dot Structures

• NH2F Amino Fluoride In this molecule,
  nitrogen is the central atom.
• Rule 1 Number of electrons 5 (2 x 1) 7
  14 7 pairs

Rule 2 Rule 3 Rule 4
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Drawing Lewis-Dot Structures
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Drawing Lewis-Dot Structures

• Polyatomic molecules with central atoms below the
  second row ten
• In this compound there are 10 valence electrons
  on bromine this is called an expanded octet.
  The extra pairs go into unfilled d orbitals.

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Example 2 Drawing Lewis-Dot Structures

• Draw electron-dot structures for
• C3H8 H2O2 CO2 N2H4
• CH5N C2H4 C2H2 Cl2CO
• H3S HCO3

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Resonance Structures

• How is the double bond formed in O3?
• The correct answer is that both are correct, but
  neither is correct by itself.

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Resonance Structures

• When multiple structures can be drawn, the actual
  structure is an average of all possibilities.
• The average is called a resonance hybrid. A
  straight double-headed arrow indicates resonance.

 
O
O
O
O
O
O
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Resonance Structures

• The nitrate ion, NO3, has three equivalent
  oxygen atoms, and its electronic structure is a
  resonance hybrid of three electron-dot
  structures. Draw them.

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Formal Charge

• Formal Charge Determines the best resonance
  structure.
• We determine formal charge and estimate the more
  accurate representation.

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Example 3 Formal Charge

• Calculate the formal charge and determine the
  most favorable of the following electron dot
  structures
• SO2 NO3 NCO N2O O3 CO32

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Example 4

• What is the overall formal charge of the
  following structure?
• -2
• -3
• -1
• 0

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Example 5 Ionic Radii of Ions

• Compare ionic radii
• Fe Fe3
• Cl Cl-