Polarity, Lewis Structures, and Resonance

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Polarity, Lewis Structures, and Resonance

• Sections 8.4-8.6

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Bond Polarity

• Ionic and covalent bonding is not black and white
• Sharing is not usually equal
• Its an electron tug of war
• Must look at electronegativity to determine how
  equally electrons are shared

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Electronegativity

• The ability of an atom in a molecule to attract
  electrons to itself
• Developed by Linus Pauling
• Gave values for all elements based on
  thermochemical data

Linus Carl Pauling (1901-1994)
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Electronegativity

• On the periodic chart, electronegativity
  increases as you go
• from left to right across a row.
• from the bottom to the top of a column.

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Electronegativities of the Elements
Cs (EN 0.7) is least electronegative element
F with EN 4.0 is most electronegative element
Au is at the peak of an island of
electronegativity, and is most electronegative
metal
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Electronegativity and Bond Polarity

• Electronegativity tells us what kind of bonding
  we have, i.e. whether it is ionic or covalent.
  The greater the difference in EN between the two
  elements forming the bond, the more ionic is the
  bond. Typical ranges for EN differences are
• EN difference bonding type Example EN
  difference
• range
• _________________________________________________
  _________________________
• gt 2.0 Ionic LiF 4.0-1.0 3.0
• 0.5-2.0 polar covalent HF 4.0-2.1 1.9
• lt0.5 nonpolar covalent F-F 4.0-4.0 0.0

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Polar Covalent Bonds

• When two atoms share electrons unequally, a bond
  dipole results.
• The dipole moment, ?, produced by two equal but
  opposite charges separated by a distance, r, is
  calculated
• ? Qr
• It is measured in debyes (D).

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Polar Covalent Bonds

• The greater the difference in electronegativity,
  the more polar is the bond.

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Bond Types and Nomenclature

• Naming ionic and covalent compounds

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Naming Ionic Compounds

• 1. Name cation first, then anion
• Cation name of the element
• Ca2 calcium
• Anion root -ide
• Cl? chloride
• CaCl2 calcium chloride

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Naming Ionic Compounds(continued)
If the metal is a transition metal

• Transition metals may form more than one cation
• Use Roman numeral in name
• PbCl2
• Pb2 is cation
• PbCl2 lead (II) chloride

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Naming Ionic Compounds(continued)

• If a polyatomic ion is present just use its name
• Ex NaCN sodium cyanide

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Naming Molecular Compounds

• 1st element in the formula is named first
• 2nd element named as if it were an anion
• Greek prefixes denote how many atoms of element
  are present
• Do not use mono- for the first element
• P2O5 diphosphorus pentoxide

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Greek prefixes cont.

• 11 hendeca-12 dodeca-13 triskaideka-14
  tetradeca-15 pentadeca-

• 16 hexadeca-17 heptadeca-18 octadeca-19
  enneadeca-20 icosa-

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Lewis Structures

• Representations of molecules showing all
  electrons, bonding and nonbonding.

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Writing Lewis Structures

• PCl3

• Find the sum of valence electrons of all atoms
• - For anions, add 1 electron for each negative
  charge.
• For cations, subtract 1 electron for each
  positive charge.

5 3(7) 26
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Writing Lewis Structures

1. The central atom is the least electronegative
   that isnt hydrogen. Connect the outer atoms to
   it by single bonds.

Keep track of the electrons 26 ? 6 20
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Writing Lewis Structures

1. Fill the octets of the outer atoms.

Keep track of the electrons 26 ? 6 20 ? 18 2
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Writing Lewis Structures

1. Fill the octet of the central atom.

Keep track of the electrons 26 ? 6 20 ? 18
2 ? 2 0
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Writing Lewis Structures

• If you run out of electrons before the central
  atom has an octet
• form multiple bonds until it does.

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Formal Charge

• Then assign formal charges.
• For each atom, count the electrons in lone pairs
  and half the electrons it shares with other
  atoms.
• Subtract that from the number of valence
  electrons for that atom The difference is its
  formal charge.

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Writing Lewis Structures

• The best Lewis structure
• is the one with the fewest charges.
• puts a negative charge on the most
  electronegative atom.

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Resonance

• This is the Lewis structure we would draw for
  ozone, O3.

-
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Resonance

• But this is at odds with the true, observed
  structure of ozone, in which
• both OO bonds are the same length.
• both outer oxygens have a charge of ?1/2.

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Resonance

• One Lewis structure cannot accurately depict a
  molecule such as ozone.
• We use multiple structures, resonance structures,
  to describe the molecule.

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Resonance

• Just as green is a synthesis of blue and yellow
• ozone is a synthesis of these two resonance
  structures.

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Resonance

• In truth, the electrons that form the second CO
  bond in the double bonds below do not always sit
  between that C and that O, but rather can move
  among the two oxygens and the carbon.
• They are not localized, but rather are
  delocalized.

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Resonance

• The organic compound benzene, C6H6, has two
  resonance structures.
• It is commonly depicted as a hexagon with a
  circle inside to signify the delocalized
  electrons in the ring.