Lewis Dot Structures
and
Molecular Geometry
Chapter 8 in online text
Chapter 15 in desk text
Why STUDY?
Warm-Up
What is the electron configuration of
carbon?
1s22s22p2
How many valence electrons does Carbon
have?
4
To determine the Lewis dot structure for Carbon we
look at the number of valence electrons carbon has.
Carbon has four valence electrons and we represent
electrons as DOTS.
C
Lewis Dot Structures for the first 10
elements on the periodic table.
Chemists use Lewis Dot Structures to determine
how elements are covalently bonded to each
other in a molecule.
• Covalently Bonded means to share electrons
between elements. Sharing of electrons occurs
between a nonmetal and another nonmetal.
• Single Bond- 2 electrons are shared
• Double Bond- 4 electrons are shared
• Triple Bond- 6 electrons are shared
Draw the Lewis Dot structure for BeF2.
1. Count the number of valence electrons you have to work with.
Be
F
Total :
valence electrons
2. Put the least electronegative element in the center (remember the trend of E.N)
**One exception: we never put Hydrogen in the center, because it can only make one
bond.
3. Use pairs of electrons to form bonds between all atoms. (2 electrons make one bond)
4. Use the remaining electrons to complete the octet rule for each element. (Exception:
Be disobeys the octet rule it only has two valence electrons)
5. If you run out of electrons, you must introduce multiple bonds to maintain the octet
rule. (BeF2 does not have multiple bonds-we will do more examples of this)
Let’s try to draw a Lewis Dot Structure of CO2 using the Rules
1. Count the number of valence electrons you have to work with.
C
O
Total :
valence electrons
2. Put the least electronegative element in the center (remember the trend of
E.N!!!?)
O
C
O
3. Use pairs of electrons to form bonds between all atoms. (2 electrons make
one bond)
4. Use the remaining electrons to complete the octet rule for each element.
5. If you run out of electrons, you must introduce multiple bonds to maintain
the octet rule.
Let’s try NH3
Try these with a partner….
H
CH4
H
C
H
H
H2O
H
HCN
C
N
Valence Shell Electron Pair Repulsion
VSEPR
Electron pairs surrounding an atom want to
be as far apart from each other as possible.
This gives rise to different molecular
shapes.
The simplest shape is a LINE
Alkali Metals and Halogens make a line
because they can only bond once.
Groups 1 and 17 make a line
Let’s look at another simple molecular shape
The electron pairs are the farthest
apart they can be.
This molecule is LINEAR
The center atom bonds twice.
Elements in group 2 make linear structures.
How is the Be atom able to covalently bound to the fluorines?
Hybridization of the orbitals explains this.
Two or more of the orbitals of the central atom must mix to form
new orbitals of equal energy so bonding can take place.
Fill in valence electrons
Hybrid mixing
2p
s
p
A linear molecule
has sp hybridization
2s
Trigonal Planar Shape
BH3
Draw Lewis Structure:
This arrangement lets all the electron pairs be
the furthest apart from each other.
Group 13 makes a trigonal planar shape
Draw electron diagram for B
Hybrid mixing
2p
2s
s
p
p
A trigonal planar
molecule has a sp2
hybridization
Tetrahedral
What does the
word ‘tetra’ mean?
CH4 Methane
Structure:
This arrangement lets all the electron pairs be the furthest
away from each other.
Group 14 makes tetrahedral shapes.
Draw the electron
diagram of Carbon
What do you think is the hybridization
of tetrahedral molecules??
2p
2s
Trigonal Pyramidal
NH3
This arrangement lets all the electron pairs be the
furthest apart from each other. With one lone pair.
This is similar to the tetrahedral arrangement ,but has
one lone pair.
Group 16 makes Trigonal Pyramidal Shapes
What is the hybridization of NH3( trigonal pyramidal)
Fill in valence electrons for Nitrogen
2p
2s
Bent shape
H2O
This arrangement lets all the electron pairs be the furthest
apart from each other. With two lone pairs.
What is the hybridization of the bent molecule? Do you
think it will be similar to tetrahedral?
Group 15 makes bent shapes.
What is the hybridization of H20?
(bent shape)
Fill in valence electrons for oxygen
2p
2s
VSEPR Theory Summary
Group 2
Group 13
Group 14
Group 15
Group 16
Shape
Linear
#of atoms
2
Hybridization
sp
Examples
CO2
Trigonal
Planar
3
sp2
BH3
Tetrahedral
4
sp3
CH4
Trigonal
Pyramidal
3 with one
lone pair
sp3
NH3
Bent
2 with two
lone pairs
sp3
H2O
Polarity of Molecules
We can determine if a molecule is polar or nonpolar by the
molecule’s shape.
In some covalent bonds electrons are not shared equally
between the elements.
The more electronegative element ‘WINS’ the electrons.
The charge difference across the bond as well as the shape of
the molecule results in a POLAR Molecule.
Polarity of Water
Lets take water for example:
Polar molecules act as tiny
dipoles because they have
un even charges.
A dipole is created by equal
but opposite charges that
are separated by a short
distance.
Direction of dipole is from
positive end to more
negative end (more
electronegative element)
Because WATER is a BENT Molecule the polarities of
each bond add together to make this molecule highly
POLAR
What about NH3 Trigonal Pyramidal
POLAR OR NONPOLAR?
The dipoles of
the three bonds
are additive
combining to
form an overall
net dipole
Because NH3 is
trigonal
pyramidal shape
it is POLAR
CH4
CO2
The individual bond polarities extend
equally and symmetrically in different
directions canceling out .
OVERALL the molecules are NONPOLAR.
What about CH3Cl?
NonPolar or Polar?
Try with a partner…
Cl
H
H
H
POLAR