Notes #10
Lewis Structures
Valence Electrons
• Valence electrons are those electrons in the
highest principle energy level (n).
• Only these outer electrons are able to react.
• How many valence e- does phosphorous have?
• Argon?
Valence Electrons
• The number of valence e- is the same as the
number for the “A” groups.
Write it
down on
your own
chart!!!
Valence Electrons
• How many electrons do most atoms want in
their “valence shell”?
• The Octet Rule – most atoms strive for 8 e- in
their outer principle energy level.
• H, Li, Be, and B don’t follow the rule. Why
might that be?
Lewis Structures
• A Lewis Dot Structure is a way to symbolically
represent the valence electrons of atoms.
• G. N. Lewis discovered the
covalent bond.
• He also coined the term
“photon”.
Lewis Structures
• 1) Each dot represents one electron.
• 2) Each line represents two shared electrons in
a covalent bond.
Lewis Structures
• Imagine there is a square around each
element’s symbol.
• Add dots to each side before doubling up.
Al
S
Lewis Structures
• Draw magnesium’s Lewis dot structure.
• Draw phosphorus's Lewis dot structure.
• Draw krypton’s Lewis dot structure.
Mg
P
Kr
Lewis Structures
Lewis Structures
•
•
•
•
•
How to draw Lewis structures for molecules:
OCl- ion (bleach)
1) Count the valence electrons for each atom.
Add e- for anions (-).
Subtract e- for cations (+).
OCl
O
Cl
(14
+ 1 e-
e)
Lewis Structures
OCl- (14 e-)
• 2) Draw a skeleton
structure with lines for shared e-.
• 3) Add e- around atoms until all (14) are used
and each has 8 e- surrounding.
8 e-
O Cl
8 e-
Note that each atom is surrounded by 8 e(Each atom has an “octet”)
Lewis Structures
• Three possibilities for central atom:
• 1) Just enough e-. Each atom gets 8, and H
gets 2.
H2O (8 e-)
H O H
Lewis Structures
• 2) Not enough e-. Move lone pairs (dots) to
make double or triple bonds until all atoms have
8 e-.
8 e-
CO2 (16
e)
O C O
8 e-
8 eNote: All atoms now have octets.
Lewis Structures
• 3) Too many e-. Atom may have “expanded
octet” (more than 8). Put extra e- around
central atom.
XeF4 (36
F
F
Xe
F
F
e)
We still
need 4
more e-!!!
Lewis Structures
• Helpful tips:
• C is always a central atom w/ 4 bonds.
C
C
• O usually makes 2 bonds.
O
O
• H and the halogens are terminal atoms that
make one bond.
H
F
• The first atom in the formula is usually the
central atom.
Practice
• Draw the Lewis structure for the sulfite ion,
SO322-
SO3 (26 e )
O
OSO
Practice
• Draw the structure for methanol, CH3OH
CH3OH (14
H
H C O H
H
e)
Practice
• Draw the Lewis structure for the nitrate ion,
NO3-
-
NO3 (24
O
O N O
e)
But wait! We
have 24 e- but N
only has 6 esurrounding it.
Whatever shall
we do?
HOMEWORK
• CW#23
• H/O Lewis #1-8, 17, 18
Notes #9b
Molecular Geometry
H2O Crisis!
• Take a look at water’s e- configuration and
orbitals.
• The prediction is that the covalent bonds with
hydrogen will be at right angles.
• In reality…
H2O Crisis!
H2O Crisis!
• We say that oxygen’s atomic orbitals have
morphed into hybridized orbitals.
• One s orbital + three p orbitals = four sp3
orbitals.
Predicting Shapes
• Molecular shapes predicted using VSEPR
theory.
• All e- pair try and get as far away from each
other as possible.
• You MUST draw the Lewis structure before the
molecular shape.
Predicting Shapes
• Predicting the bond angles
• Add up the number of “forces” surrounding
central atom.
• Lone pairs and bonds count as one force.
• Double and triple bonds are also one force.
Predicting Shapes
• Three Basic Angles.
• 2 Forces: Linear molecule,
180° angles around
central atom
• 3 Forces: Trigonal planar,
120° angles
• 4 Forces: Tetrahedral,
109° angles
Predicting Shapes
• You just saw 3 possible angles, but they can
make 5 possible molecular shapes.
• Shape based on 3D arrangement of atoms.
• Lone pair e- influence shape, but are not seen.
Predicting Shapes
• What is the molecular geometry for BeF2?
• 180°, linear, non-polar
Predicting Shape
• Determining Polarity
• Molecules are polar when more electrons are on one
side than the other.
Partial +
Partial -
• If symmetrical, usually nonpolar
• If bent, unshared e-, or different terminal atoms,
usually polar.
Predicting Shape
• Polar or Nonpolar?
Practice
• What is the molecular geometry of BF3?
(angle, shape, polarity)
• 120° angle, trigonal planar, nonpolar
Practice
• Molecular geometry of carbon tetrafluoride, CF4
(angle, shape, polarity)
• 109° angles, tetrahedral, nonpolar
Practice
• A view of the hybrid orbitals and covalent bonds
Practice
• Geometry of water, H2O (angle, shape, polarity)
• 105° angle, bent, polar
Practice
• More views of water
Practice
• Geometry of ammonia, NH3
• 107° angles, trigonal pyramidal, polar
Tidbits
• Expanded Octet: Six e- forces makes an
octahedron.
Tidbits
• If two of those forces are lone pairs, you get
square planar.
Tidbits
• It’s all broken down on this handout. It TELLS
you the ANSWERS!
Summary
• 2 forces: angle is 180°
can only be linear
• 3 forces: angles are 120°
can be trigonal planar or bent (look for lone
pairs)
• 4 forces: angles are about 109°
can be tetrahedral, trigonal pyramidal, or bent.
• 5 or 6 forces: angles are 90° or 120°
can be trigonal bipyramidal, see-saw,
octahedral, T-shaped, trigonal planar, or square
planar (LOOK AT THE SHEET)