Journal Entry
Draw Lewis structures for the following
molecules:
a) SCl2
b) CNF
Molecular Geometry and VSEPR Theory
• Whenever the atoms of a molecule bond together,
the molecule takes on a particular shape.
• Molecular geometry refers to the three-dimensional
arrangement of a molecule’s atoms in space.
• VSEPR theory is the model used to explain the
shapes of molecules.
• VSEPR – Valence Shell Electron Pair Repulsion
Theory.
• The main idea of this theory is that electron pairs
on the central atom of a molecule will position
themselves in such a way as to minimize repulsion.
• There are two types of electron pairs:
– bonding pairs (shared)
– nonbonding or lone pairs (unshared)
• Example – CH4
– 4 electron pairs (tetrahedral electron
geometry)
– 4 bonding / 0 nonbonding pairs
– molecular geometry = tetrahedral
– bond angles are 109.5°
• Example – NH3
– 4 electrons pairs (tetrahedral electron
geometry)
– 3 bonding / 1 nonbonding
– molecular geometry = trigonal pyramidal
– bond angles are 107°
– Why are bond angles less?
• Example – H2O
– 4 electron pairs (tetrahedral electron
geometry)
– 2 bonding / 2 nonbonding
– molecular geometry = bent
– bond angle = 104.5°
– Why are bond angles less?
– Unshared pairs of electrons repel the
adjacent bonding pairs more strongly than
the bonding pairs repel each other.
VSEPR and Multiple Covalent Bonds
• For the VSEPR model, multiple bonds are counted
as one bonding pair when determining the shape of
the molecule.
• Example – CH2O
– 3 electron pairs (trigonal planar electron
geometry)
– 3 bonding / 0 nonbonding
– molecular geometry = trigonal planar
– bond angles = 120°
• Example – NOCl
– 3 electron pairs (trigonal planar electron
geometry)
– 2 bonding / 1 nonbonding
– molecular geometry = bent
– bond angles = approx. 120°
• Example – CO2
– 2 electron pairs (linear electron geometry)
– 2 bonding / 0 nonbonding
– molecular geometry = linear
– bond angles = 180°
Bonding Pairs
Nonbonding Pairs
Shape
Example
2
0
Linear
CO2
3
0
Trigonal Planar
CH2O
2
1
Bent
NOCl
4
0
Tetrahedral
CH4
3
1
Trigonal Pyramidal
NH3
2
2
Bent
H2O