Drawing Lewis structures
1. Calculate the total number of valence electrons
in the compound.
2. Choose the central atom and place the
remaining atoms symmetrically around the
central atom.
3. Place one electron pair (or a line) between each
pair of bonded atoms.
4. Complete the octet of each atom (duplet for H)
by placing the remaining valence electron as
electron pairs around the atoms.
5. If there are not enough electrons form multiple
bonds.
Shapes of Molecules
• Electron pairs surrounding an atom repel each
other. This is referred to as Valence Shell
Electron Pair Repulsion (VSEPR) theory.
• The electron pair geometry indicates the
arrangement of bonding and nonbonding electron
pairs around the central atom.
• The molecular shape gives the arrangement of
atoms around the central atom as a result of
electron repulsion.
How To Use VSEPR Model?
1. Draw the Lewis structure
2. Identify the central atom and assign the letter A
to the central atom.
3. Count the total number of electron pairs
(bonding and nonbonding) around the central
atom. Each lone pair will be assigned the letter
E, and each bonding pair will be assigned the
letter X. Treat a multiple bond equivalent of a single
electron pair
3. Arrange the electron pairs in a way to minimize
e--e- repulsion. The lone pairs must be as far as
possible from one another and from bond pairs.
Shape of Molecules AXnEm
A is the central atom
n is the number of bonding pairs,
m the number of non bonding pairs
n
m
Example
Shape
2
0
BeH2
Linear
3
0
BH3
Trigonal Planar
4
0
CH4
Tetrahedradral
2
1
HNO
Bent
3
1
NH3
Trigonal Pyramidal
2
2
H2O
Bent
Linear Molecule
Bent Molecule
Tetrahedral
Molecule
Trigonal
Pyramidal
Molecule
Tetrahedral Molecules
• Methane, CH4, has four pairs of bonding electrons
around the central carbon atom.
• The four bonding pairs (and therefore atoms) are
repelled to the four corners of a tetrahedron. The
electron pair geometry is tetrahedral.
• The molecular
shape is also
tetrahedral.
Trigonal Pyramidal Molecules
• In ammonia, NH3, the central nitrogen atom is
surrounded by three bonding pairs and one
nonbonding pair.
• The electron pair geometry is tetrahedral and the
molecular shape is trigonal pyramidal.
Bent Molecules
• In water, H2O, the central O atom is surrounded
by two nonbonding pairs and two bonding pairs.
• The electron pair geometry is tetrahedral and the
molecular shape is bent.
Linear Molecules
• In carbon dioxide, CO2, the central C atom is
bonded to each oxygen by two electron pairs (a
double bond).
• According to VSEPR, the electron pairs will repel
each other, and they will be at opposite sides of
the C atom.
• The electron pair geometry and the molecular
shape are both linear.
Summary of VSEPR Theory
Molecular Polarity
• In water, the molecule is not linear and the bond dipoles
do not cancel each other.
• Therefore, water is a polar molecule.
Polar Bond
The overall polarity of a molecule depends on its molecular
geometry
Conclusions Continued
• VSEPR Theory can be used to predict the shapes
of molecules.
• The electron pair geometry gives the arrangement
of bonding and nonbonding pairs around a central
atom.
• The molecular shape gives the arrangement of
atoms in a molecule.
• The molecular shape gives information about the
polarity of the molecule.