Lewis Structures Resonance Exceptions to the Octet Molecular Geometry Sigma and Pi bonds

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Unit 8

• Lewis Structures Resonance Exceptions to the
  Octet Molecular Geometry Sigma and Pi bonds

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Lewis Structures general rules to follow

• Many molecular (covalent) compounds follow the
  octet rule which assumes that when an atom is
  correctly bonded, it will have 8 electrons either
  shared or otherwise around it.
• .. ..
• Cl Cl Cl2
• .. ..
• Electrons not shared are called lone pairs and
  these have electron repulsion that affect the
  shape of the molecule.
• When the electron pairs exactly offset each
  other, they have the largest bond angle , 180
  degrees

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Calculate the number of bonds you need

• Consider the compound, CH2Cl2, Counting up
  valence electrons you get C 4 H1 x 22 and
  Cl2 x 714 with a total of 20 valence electrons
• Subtract from this the number of electrons needed
  for all atoms to fulfill the octet (C8 H2 x
  24 Cl8 x 216 total needed is 28 divide the
  result by 2 to get the of bonds
• 28-20/2 8/2 4
• 4 the number of bonds on the central atom
• You should assume carbon is always the central
  atom because it usually has the lowest
  electron-negativity, you should also assume that
  hydrogen is a terminal atom (only bonds to one
  other atom) not a central atom.

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Draw the Lewis Structure

• Draw these structures, for each single bond
  sharing one electron from each atom, add in the
  dots to complete the valence for each atom
  (remember hydrogen doesnt get ANY EXTRA)
• a. HF d. CH4
• b. N2 e. CF4
• c. NH3 f. NO
• Can you predict if there will be a dipole and if
  so in what direction?

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Lewis Structures general rules to follow

• Many molecular (covalent) compounds follow the
  octet rule which assumes that when an atom is
  correctly bonded, it will have 8 electrons either
  shared or otherwise around it.
• .. ..
• Cl Cl Cl2
• .. ..
• Electrons not shared are called lone pairs and
  these have electron repulsion that affect the
  shape of the molecule.
• When the electron pairs exactly offset each
  other, they have the largest bond angle , 180
  degrees

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Is the molecule polar or non-polar? ( we are not
talking about the bonds here)

• IF octet rule is obeyed AND all
• the surrounding bonds are the same
• even if very polar then the
• molecule is NONpolar since all the
• dipole moments cancel each other
• out.

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What about multiple bonds?

• If all you have is a single bond this is called a
  sigma bond(s)
• If there is double bond, then the first bond is a
  sigma bond, s, the second bond is called a pi
  bond,p.
• If there is a triple bond then there is one sigma
  bond and two pi bonds, p.
• There are only certain atoms that can make
  multiple bonds, C, N, O, P, S, remember it as
• C-NOPS

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Comments about the Octet Rule

• Second row elements, like C, N, O, P, S should
  always be assumed to obey the octet rule.
• Second row elements Be and B often can be found
  to have fewer than eight electrons
• The second row electrons never violate the octet
  rule they have 2s and 2p and can accommodate no
  more than eight electrons
• Third period and heavier elements often follow
  the octet rule but they can exceed it by using
  their empty valence d orbitals
• PROCEDURE When writing the Lewis Structure for a
  molecule, satisfy the octet for each element
  first, if there are still electrons remaining,
  then place them on the central element which
  should have available d orbitals (3d or higher)

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How do you tell if there are multiple bonds?

• Once all the electrons have been placed Around
  each atom, if the central atom is not yet
  surrounded by four electron pairs, convert one or
  more terminal atom lone pairs to pi bond pairs.
• Try Oxygen gas, O2, and carbon dioixde and sulfur
  dioxide

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VSEPR determines a molecule's shapeValence Shell
Electron Pair Repulsion Theory

• Molecular shape changes with the numbers of s
  bonds plus lone pairs about the central atom.

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What do they look like?
180 linear and of course
planar
120 trigonal planar
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120 90 trigonal bipyramidal

• 109.5 tetrahedral

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90 octahedral
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Try another, and another and another and so forth

• Drawing Lewis Structures (predicting molecular
  shape)
• Give the Lewis structure for each of the
  following, is there a dipole? What about polarity
  of the molecule?
• a. HF d. CF4
• b. CO2 e. ICl2
• c. NH3 f. HCN

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Solution

• H-F with the remaining electrons around the F
  the noble gas, octets are fulfilled. A linear
  shape is made but there is a dipole from H to F
  and this makes the molecule polar.
• OCO when remaining valence electrons are placed
  around the oxygens the octets are fulfilled, a
  linear shape is made and there is no dipole
  because they are cancelled out

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Solution

• . .
• H N H the extra electrons on the N bend the
  H
• H downward, making a
  trigonal
• pyramidal shape, the
  dipole is generally downward thus making the
  molecule polar.
• d. F
• F- C F The valence electrons fill the
  octet
• F around each F making a
  tetrahedral shape, there is no dipole due to
  canceling, and thus it is a non-polar molecule

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Solution

• Cl I Cl When extra valence are arranged
  around the Cls and the remaining are placed on
  the central atom (dont forget the one less
  electron due to the 1 charge), this makes a
  bent or V-shaped molecule the dipole is
  generally downward and the molecule is polar.
• H- CN putting extra valence on the N gives a
  linear shaped molecule but the dipole towards the
  N means it is a polar molecule.

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Exceptions to the octet usually occur in the
central atom of the 3rd and higher periods

• Place lone pairs about each terminal atom
  (EXCEPT H) to satisfy the octet rule. Left over
  pairs are assigned to the central atom.
• If the central atom is from the 3rd or higher
  period, it can accommodate more than four
  electron pairs.
• Calculate the number of bonds for phosphorus
  penta-chloride P 5 Cl7 x 5 35 5 35 40
• Valence if all atoms have fulfilled their octet
  P8 and Cl5 x 8 40 so 48.
• 48- 40/ 2 4 bonds but thats not enough for the
  five Cl in the formula this means it is an
  EXCEPTION!!!
• Write the Lewis structure for PCl5.

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Solution
See this as two axial (north and south) and three
equatorial chlorines, this represents the
farthest apart electron configuration possible
What Shape is this?
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Other Exceptions to the octet

• when there are an odd number of valence electrons
• Be and B are always electron deficient, not
  enough to fill the octet.
• Sometimes there is uneven sharing and only the
  central atom has to share but they still end up
  following the octet rule

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Try these problems

• Write the Lewis structure for each
• molecule or ion.
• a. ClF3 b. XeO3 c. RnCl2
• d. BeCl2 e. ICl4-

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Solutions

• The chlorine atom (third row) accepts
• the extra electrons. Shape? Dipole?
• Polarity?
• b. All atoms obey the octet rule.
• Sometimes there is uneven
• sharing, when the central atom
• shares but the others dont. Shape?
• Dipole? Polarity?

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Solutions

• Randon, a noble gas, accepts
• the extra electrons and bonds
• with the Cl. Shape? Dipole?
• Polarity?
• d. Beryllium is electron deficient. Shape?
  Dipole? Polarity?

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Solutions

• e. Dont forget the extra electron from the ion
  when you calculate the of bonds. Iodine exceeds
  the octet rule. Brackets are used to show the
  ion charge.