Download 10 • Bonding

South Pasadena • Honors Chemistry
Name
10 • Bonding
Period
10.2
NOTES
–
Date
MOLECULAR SHAPES
The Lewis Dot Structures for covalent compounds are two dimensional representations and help us determine
how atoms are connected. However, they do not describe well the arrangements of these connections to form
three-dimensional shapes of the molecules.
Valence Shell Electron Pair Repulsion Theory (VSEPR)
1. Determine the steric number: The bonds formed with the atom and the lone electron pairs. VSEPR states that
electron groups want to be arranged as far from each other as possible.
4
3
2
Steric Number
Overall Geometry
Bond Angles
Tetrahedral
109.5°
Trigonal Planar
120°
Linear
180°
2. Determine how the bonding electron groups are arranged. We use the notation ABxEy to indicate that atom A
has x bonding electron groups and y non-bonding electron groups (lone electron pairs).
4
3
2
Steric Number
AB4 – Tetrahedral
AB3 – Trigonal Planar
AB2 – Linear
0 Lone Pairs
AB3E1 – Trigonal Pyramidal
AB2E1 – Bent
1 Lone Pair
AB2E2 – Bent
2 Lone Pairs
Example 1a. Draw the Lewis Structures, determine the molecular shape, and predict the bond angles for the
following compounds: CH2Cl2, NH3, and H2O.
H
H C Cl
H
H N H
O H
H
Cl
Tetrahedral, 109.5°
Polar compound
(polar bonds do not cancel out).
Trigonal pyramidal, 107°
Polar compound
(polar bonds do not cancel out)
Bent, 104.5°
Polar compound
(polar bonds do not cancel out)
Example 1b. Draw the Lewis Structures, determine the molecular shape, and predict the bond angles for the
following compounds: BF3, BeH2, and Br2.
F
F
B F
Trigonal planar, 120°
Non-polar compound
(polar bonds cancel out)
Br Br
H Be H
Linear, 180°
Non-polar compound
(no polar bonds)
Diatomic
Non-polar compound
(no polar bonds)
Molecular Polarity – While we can determine whether a bond between non-metals is polar, it’s the
polarity of the overall molecule that explains many observed properties of a covalent compound.
Polar Compound – when charges are unevenly distributed in the molecule.
A molecule is polar if:
(1) The compound needs to have polar covalent bonds
(2) these polar bonds are arranged in a way that their poles do not cancel out (i.e. there is asymmetry in the
pull of these bonds).
Example 2
 Determine whether the following compounds form polar or non-polar molecules: Cl2, HCl, CO2
Cl Cl
Diatomic
Non-polar compound
(no poplar bonds)

O C O
H Cl
Diatomic
Polar compound
(Polar bonds do not cancel out)
Linear, 180°
Non-polar compound
(Polar bonds cancel out)
Determine whether each of the compounds in Example 1 form molecules that are polar.
Summary for Describing Covalent Compounds
Draw the
Lewis Dot
Structure
Find the
Steric
Number
Determine the
Molecular
Shape
Are there Polar
Covalent Bonds?
AB4
Tetrahedral
Steric Number 4
Overall Geometry:
Tetrahedral
Bond Angles: 109.5°
••
AB3E1
Trigonal
Pyramidal
••
AB2E2
Bent
Steric Number 3
Overall Geometry:
Trigonal Planar
Bond Angles: 120°
Steric Number 2
Overall Geometry:
Linear
Bond Angles: 180°
Non-Polar
Molecule
Do Polar Bonds
Cancel Out?
Polar
Molecule
••
AB2
Linear
YES
NO
NO
••
AB3
Trigonal
Planar
AB2E1
Bent
Predict if the
Molecule is
Polar
YES
Non-Polar
Molecule