11.6 Lewis Structures

1

• 11.6 Lewis Structures

2
Electron Shells and Sizes of Atoms

• The outermost, or highest occupied shell, is
  called the valence shell.
• The electrons that occupy the valence shell are
  called valence electrons

Each of these atoms has one valence electron
valence e-s
B He 2s22p1 3 C He 2s22p2 4 N He
2s22p3 5 O He 2s22p4 6 F He 2s22p5 7
Li He 2s1 Na Ne 3s1 K Ar 4s1 Rb
Kr 5s1
3
Lewis Symbols

• The electrons involved in chemical bonding are
  the valence electrons.
• American chemist, G.N. Lewis, determined a method
  to show valence electrons on atoms and track them
  in compounds.
• The Lewis symbol for an element consists of the
  element symbol plus a dot for each valence
  electron.
• Consider the element sulfur

Note by convention, electrons are usually
distributed right to left then top to bottom.
Ne3s23p4
4
Lewis Symbols
5
Lewis Symbols

• Atoms gain or lose enough electrons to achieve
  the same number as the nearest noble gas.
• All noble gases (except He) have eight valence
  electrons so atoms tend to lose or gain electrons
  until they are surrounded by eight electrons.
• This is known as the octet rule

2-
S Ne3s23p4
S2- Ne3s23p6
-
..
.
Cl Ne3s23p5
Cl- Ne3s23p6
Note that ions are placed in brackets
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Lewis StructuresCovalent Compounds

• Covalent bonds can be represented with Lewis
  structures

H H ? H H
Covalent bonds are formed using the valence
electrons of each atom
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Lewis StructuresMolecular Compounds

• When atoms form covalent bonds, they share
  electrons.
• Through the sharing of electrons, each element
  gains a noble gas configuration!

H H ? H H
?
?
?
?
?
?
?
?
?
?
?
?
With the exception of hydrogen, covalent bond
formation allows each element to be surrounded by
eight electrons in a Noble gas configuration
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Lewis StructuresMolecular Compounds

• Each pair of electrons that connect two atoms is
  often represented as a line between the two
  atoms
• Molecules with more than two atoms are
  represented similarly

Electrons not involved in the bonding are called
the lone pairs
NH3
H2O
CCl4
These are the bonding pairs
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• 11.7 Lewis Structures of Molecules with Multiple
  Bonds

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Lewis StructuresMultiple Bonds

• Each pair of shared electrons constitutes a bond
• Many molecules require the sharing of more than
  two electrons to form an octet around each atom.

..
..
..
..

O C O ? OCO
..
..
..
..

..
..
..
..
OCO ?
..
..
Carbon dioxide, CO2, molecules contain two bonds
between carbon and each oxygen. There are two
double bonds in this compound.
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Lewis StructuresMultiple Bonds
N N ? N ? N
Nitrogen, N2, molecules contain three bonds. We
call this a triple bond
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Drawing Lewis Structures

• Sum all valence electrons
• Draw diagram with all atoms joined by a single
  bond. The central atom is usually the one written
  first in the formula. (note that hydrogen is
  never the central element)
• Subtract bonded electrons from the total. (these
  are the leftovers)
• Distribute leftovers around outside atoms first,
  then central atom so that all are surrounded by
  eight electrons (note that hydrogen is an
  exception and will only be surrounded by two
  electrons)
• If there are not enough electrons to give the
  central atom an octet, try adding multiple bonds
  so that ALL atoms (except H) have an octet.

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Drawing Lewis Structures
Draw the Lewis structure for CF4
carbon 4 valence electrons for each ? 4
valence electrons Fluorine 7 valence electrons
for each ? 28 valence electrons
F
Total valence electrons 32 - electrons
required for bond - 8 Leftover 24
C
F
F
F
Leftover electrons must be distributed around
molecule starting with outer elements
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Drawing Lewis Structures
The correct Lewis structure for CF4 is
..
F
.. ..
.. ..
C
F
F
F
..
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Drawing Lewis Structures
Draw the Lewis structures for the following
compounds CH4 PCl3 SiH4 SO42- PO43-
NO2- NH4 NF3
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Resonance Structures

• Consider the Lewis structure of SO3
• There are several equal structures that could be
  drawn
• Because each one is exactly the same and valid,
  we say they are resonance structures.
• Consider the Lewis structure of NO3-

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Resonance Structures
Draw the Lewis structure for NO3- show all
resonance structures
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Lewis StructuresExceptions to the Octet Rule

• Less than an octet
• Some compounds are stable even when there are
  less than 8 electrons around the central atom
• In particular, compounds of boron (B) and
  beryllium (Be) do not require an octet
• BF3
• BeF2

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• 11.8 Molecular Structure

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Molecular Shapes
Aspirin

• The size, shape and polarity of a molecule are
  key factors in the properties of the molecule
• The reactivity of aspirin is due to its shape and
  size.
• In biological molecules, shape is very important
  for function. (The sensation of smell is due to
  size and shape of specific molecules)

Protein molecule
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Molecular Shapes

• Previously, we used Lewis structures to determine
  the electron distribution in molecules.
• Note that molecules are 3-dimensional, where
  Lewis structures are 2-D. Lewis structures do not
  directly indicate shape.
• The shape of a molecule is determined by the bond
  angles, or the angle formed by the lines joining
  the nuclei of atoms in the molecule. Consider
  CCl4

109.5
109.5
CCl4 forms tetrahedron
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Molecular Shapes
Representations of molecules
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• 11.9 Molecular Structure The
• VSEPR Model

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Molecular ShapesVSEPR

• The shapes of molecules can be predicted using
  the Valence Shell Electron Pair Repulsion (VSEPR)
  model
• The valence electrons are the outermost electrons
  and determine the bonding in molecules.
• Electron pairs in a molecule will repel each
  other and they will be forced to separate as far
  as possible.

180
2 electron pairs around Be will separate by
180! The molecules shape (geometry) is linear.
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Molecular ShapesVSEPR

• Each BF3 molecule contains three electron pairs
  around the boron atom

3 pairs of electrons around B will separate by
120! The molecular geometry is trigonal planar.
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Molecular ShapesVSEPR

• Each CCl4 molecule contains four electron pairs
  around carbon

109.5
109.5
The electron domain geometry and molecular
geometry are tetrahedral
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Molecular ShapesVSEPR

• Each NH3 molecule contains four electron domains
  around carbon.

The electron domain geometry of NH3 is
tetrahedral but the molecular geometry is
trigonal pyramidal (three sided pyramid)
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Molecular ShapesVSEPR

• Each H2O molecule contains four electron domains
  around carbon.

109.5
..
..
109.5
The electron domain geometry of H2O is
tetrahedral but the molecular geometry is bent
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Molecular ShapesVSEPR
Lewis structure
Count electron domains
Electron domain geometry
Molecular geometry
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Molecular ShapesVSEPR

• Determine the electron domain and molecular
  geometries for
• SnCl3-
• O3
• NH4

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• 11.10 Molecular Structure
• Molecules with Double Bonds

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Molecular ShapesVSEPR

• Carbon dioxide contains two sets of double bonds.
• Each set of double bonds acts as one unit.
• We can think of each region as a separate
  electron density cloud. The clouds repel each
  other and separate by 180?.

180
2 regions of electron density around C will
separate by 180! The molecules shape (geometry)
is linear.
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Molecular ShapesVSEPR

• Note that even though non-bonded and bonded
  electrons repel, the molecular geometry relates
  only to the bonded atoms.
• Each NO2- ion contains three electron domains
  around the boron atom

..
O

..
120
N
3 regions of electron density around N will
separate by 120! The electron domain geometry is
trigonal planar and the molecular geometry is
bent.
120
120
O
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Molecular ShapesVSEPR
Predict the structure of NO3-