Lewis Structures and Molecular Shapes

```Lewis Structures and Molecular
Shapes
Drawing Lewis Structures
• Determine from formula if ionic or covalent
• Count the electrons
– If ionic : add valence # to charge if (-), subtract if
(+) F- = 7+1 electrons; Na+ = 1-1 electrons
Total= 8 electrons
– If molecular or covalent, add valence numbers
SO3: S=6 e O=6 e x 3 Total= 24 electrons
Draw the Skeleton Structure
• Place the least electronegative atom in the
middle (halogens, hydrogen or oxygen usually
go outside)
• Draw a dash between each atom (e- pairs)
• Distribute electrons around the structure until
all atoms (except H) have eight electrons
Satisfy Octet Rule
• Place remaining valence electrons around the
rest of the atoms (halogens, N, P etc.). The
middle atom usually has none.
• Electronegative atoms usually have lone pair
electrons if any are available.
• For odd electron species, electronegative
elements are better radicals, so they get the
odd electron
Determine electronic character
CO3-2 : non-metals, must be covalent
Electronegativity of C: 2.5
Electronegativity of O: 3.5
C must be central
Count electrons
•
•
•
•
Carbon: groupIV, has 4 valence electrons
Oxygen: group VI, has 6 valence electrons
Has a charge: add 2 electrons
Place a double bond between atoms where
the octet rule is not satisfied. This may require
removing lone pairs from an electronegative
atom connected to the deficient atom
Write the Lewis structure of the carbonate ion (CO32-).
Step 1 – C is less electronegative than O, put C in center
Step 2 – Count valence electrons C - 4 (2s22p2) and O - 6 (2s22p4)
-2 charge – 2e4 + (3 x 6) + 2 = 24 valence electrons
Step 3 – Draw single bonds between C and O atoms and complete
octet on C and O atoms.
Step 4 - Check, are # of e- in structure equal to number of valence e- ?
3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons
Step 5 - Too many electrons, form double bond and re-check # of e-
O
C
O
O
2 single bonds (2x2) = 4
1 double bond = 4
8 lone pairs (8x2) = 16
Total = 24
What are the resonance structures of the
carbonate (CO32-) ion?
-
O
C
O
O
-
O
C
O
O
-
-
-
O
C
O
O
-
Resonance describes the shortend bond length observed
In CO32- .
Structural electron pairs
• There are four structural pairs on the O in H2O
H O H
• There are three structural pairs on the N in
NO2–
O N O
Practice
• How many structural pairs on the central atoms?
O
F
O C O
F
S
F
F
F
F
F
S
F
F
F
O
Xe F
F
Practice
• How many structural pairs on the central atoms?
O
O C O
3
5
F
F
S
F
F
F
F
F
S
F
F
F
6
O
Xe F
F
5
Two structural pairs--linear
180°
Three structural pairs--trigonal planar
Four structural pairs--tetrahedral
Practice
• Find the structural-pair geometry, molecular
geometry and bond angle(s) for this:
O
O C O
Practice
O
O C O
structural pairs on central atom
3
structural pair geometry
trigonal planar
molecular geometry
trigonal planar
bond angle(s)
120°
Sigma and pi bonds
1σ+1π
1σ+2π
O O
N N
O O O
O O O
1σ+½π
Practice
How many sigma bonds and pi bonds connect each pair of
atoms in the molecule and polyatomic ion shown below?
H H
H C C C N
O
O
O
O C O
O C O
O C O
Practice
1σ
1σ
H H
H C C C N
1σ+2π
1σ+1π
O
O
O
O C O
O C O
O C O
all C-O bonds in CO3–2 are 1 σ + 1/3 π
Cl
Be
Cl
0 lone pairs on central atom
2 atoms bonded to central atom
10.1
10.1
10.1
Effect of Lone Pair on Central Atom
Total – 3 HED
Class
# of atoms
bonded to
central atom
# lone
Pairs/fr
on central
atom
Arrangement of
electron pairs
Molecular
Geometry
trigonal
planar
bent
AB3
3
0
trigonal
planar
AB2E
2
1
trigonal
planar
Effect of Lone Pair on Central Atom
Total – 4 HED
Class
AB4
# of atoms
bonded to
central atom
# lone
pairs on
central atom
4
0
Arrangement of
electron pairs
Molecular
Geometry
tetrahedral
tetrahedral
AB3E
3
1
tetrahedral
trigonal
pyramidal
AB2E2
2
2
tetrahedral
bent
O
H
H
NO2 vs CO2
Class
NO2 AB2E
Molecular Bond
Structure Angle
Bent,
Angular
O
C
O
< 120°
N
CO2 AB2
Linear
180°
O
O
BH3 vs NH3
Class
H
Molecular Bond
Structure Angle
B
BH3 AB3
NH3 AB3E
Trigonal
Planar
120°
Trigonal <109.5°
Pyramidal
H
H
N
H
H
H
Molecular Structure Depends on #
Atoms and # HED
Regions of HED around Central Atom
2
3
4
2
3
4
Linear,
180°
Angular (Bent), Angular (Bent),
<120°
<109.5°
Trigonal Planar, Trigonal Pyramidal,
120°
109.5°
Tetrahedral, 109.5°
Dipole Moments and Polar Molecules
electron poor
region
electron rich
region
H
F
δ+
δ−
10.2
10.2
Determining Polarity of Molecules
1)
2)
3)
4)
Draw Lewis Dot Structure
Determine Molecular Structure (3-D Shape)
Look for Dipoles Within Molecule
Determine if Dipoles Reinforce or Cancel
– Reinforce => Polar Molecule
– Cancel => Nonpolar Molecule
Which of the following molecules have a dipole moment?
H2O, CO2, SO2, and CH4
O
S
dipole moment
polar molecule
dipole moment
polar molecule
H
O
C
O
no dipole moment
nonpolar molecule
H
C
H
H
no dipole moment
nonpolar molecule
10.2
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