Lewis Structures
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Transcript of Lewis Structures
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Lewis Structures• H is always a terminal atom• The atom with the lowed EN is the central atom• Find the total # of valence (ions add and lose
electrons)• Bond each atom to the central atom with a single
bond• Place lone pairs around terminal atoms to complete
their octet• Add remaining electrons to central atom• Use multiple bonds to complete the octet of the
central atom
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Exceptions to the Octet Rule
• Hydrogen only gets 2• Boron is a moron, he likes 6 electrons• Expanded Valence – 3rd energy level or higher
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HYBRID ORBITALS
covalent bonds are formed by overlap of atomic orbitalsatomic orbitals on the central atom can mix and exchange their character – hybridization
ONLY BONDS ARE FORMED FROM
OVERLAP OF HYBRIDS ORBITALS ARE NOT FORMED FROM HYBRID ORBITALS
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Number of Number of electron electron
structural structural pairspairs
Atomic Atomic orbitalsorbitals
Hybrid Hybrid orbitals orbitals formedformed
GeometryGeometry
22 one s + one pone s + one p two sp two sp AX2: linear
33 one s + two pone s + two p three spthree sp22 AX3: trigonal planar
44one s + three one s + three
ppfour spfour sp33 AX4: tetrahedral
55s + three p + s + three p +
one done d five spfive sp33d d AX5
: trigonal bipyramidal
66one s + three one s + three
p + two dp + two d six spsix sp33dd22 AX6: octahedral
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practice
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2 sp Hybrids Formed From an s and p
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3 sp2 Hybrids Formed From an s and 2 p’s
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Forming sigma bonds• Three ways atoms can form a sigma bond:– 2 hydrogen atoms’ 1s orbitals overlap
– a hybrid orbital overlaps a hydrogen’s 1s orbital
– 2 hybrid orbitals overlap end-to-end
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Forming pi bonds• Pi bonds form when– atoms are already bonded with a sigma bond– both atoms have a leftover p orbital
• Pi bonds form when two atoms’ p orbitals overlap side-by-side
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NO21- Resonance Structures
next formal charge
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Delocalized pi bonding in CO3–2
• In the CO3–2 ion, one pi bond is delocalized
over three C-O connections• The C atom and all three O atoms are sp2
hybridized and have a leftover p orbital• The four p orbitals overlap side-to-side to
create the pi bonding
O
O
C OO
O
C O O
O
C O
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C6H6, Benzene an Important Molecule With Resonance
Delocalized e- due to resonance explains why the bonds in benzene
are identical.
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LINEAR
AX2 EXAMPLE BH2
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AX2 WITH DOUBLE BOND CO2
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AX3 EXAMPLE BH3
TRIGONAL PLANAR
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O
O
C O
structural pairs on central atom 3
structural pair geometry trigonal planar
molecular geometry trigonal planar
bond angle(s) 120
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AX4EXAMPLE CH4
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AX3E EXAMPLE NH3
PYRAMIDAL
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AX2E2 EXAMPLE H2S
BENT
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AX5 EXAMPLE PCl5
TRIGONAL BIPYRAMIDAL
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AX4E EXAMPLE SF4
SEE SAW
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AX3E2 EXAMPLE BF3
T SHAPE
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EXAMPLE XeF2
LINEAR
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structural pairs on central atom 5
structural pair geometry trigonal bipyrimid
molecular geometry T-shaped
bond angle(s) 90 , 180
O Xe F
F
Next resonance
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structural pairs on central atom 5
structural pair geometry trigonal bipyrimid
molecular geometry see-saw, flying goosedistorted tetrahedron
bond angle(s) 120 , 90 , 180
SF F
F
F
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EXAMPLE SeF6
OCTAHEDRAL
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EXAMPLE IF5
SQUARE PYRAMIDAL
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EXAMPLE XeF4
SQUARE PLANAR
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structural pairs on central atom 6
structural pair geometry octahedron
molecular geometry octahedron
bond angle(s) 90 , 180
S FF
FF
FF
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Polar molecules• A compound’s polarity can be measured
experimentally as its dipole moment• Polar molecules align themselves with an
electrical field
+ –
+ –
+ –
+ –
+ –+ –
+ – + –
+ – +++
+++
–––
–––
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Polar molecules• The C=O bond is polar• But the dipole
moment of CO2 is zero
• CO2 is not a polar molecule
O C O
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ExamplesExamples
SO2
• S-O bonds are polar• Geometry is bent (120°)• sp• Bond dipoles don’t cancel• SO2 is polar
SO3
• S-O bonds are polar• Geometry is trigonal
planar• sp2
• Bond dipoles do cancel• SO3 is nonpolar
O S O O S O
O
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Examples
HH22OO
• H-O bonds are polarH-O bonds are polar• Geometry is bent (109.5°)Geometry is bent (109.5°)• spsp• Bond dipoles don’t cancelBond dipoles don’t cancel• HH22O is polarO is polar
OClOCl22
• O-Cl bonds are nonpolarO-Cl bonds are nonpolar• spsp• OClOCl22 is nonpolar is nonpolar
H O H Cl O Cl
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SF4
• S-F bonds are polar• geometry is see saw• dsp3
• axial bond dipoles cancel• equatorial bond dipoles
don’t cancel• SF4 is polar
PF5
P-F bonds are polar• geometry is trigonal
bipyramid• dsp3
• axial bond dipoles cancel• equatorial bond dipoles
cancel• PF5 is nonpolar
SF F
F
FSF F
F
FF
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XeF2
• Xe-F bonds are polar• geometry is linear
– structural pair geometry is trigonal bipyrimidal
– lone pairs in equatorial– F atoms in axial– dsp3
• bond dipoles cancel• XeF2 is nonpolar
PbH2
• Pb-H bonds are nonpolar• PbH2 is nonpolar
F Xe F
H Pb H