Guided Notes: Molecular Structure
VSEPR theory
Valence-Shell Electron-Pair Repulsion Theory: Because electron pairs _________________, molecules adjust their
shapes so that valence electron pairs are as far _______________________ as possible .
Types of electron pairs
Bonding Pairs: Electrons that form ________________
Lone (unshared) Pairs: Electrons that are ___________________________; no atom trying to bond. These
are held _________________________ to the atom they are associated with than bonding pairs.
Lone Pairs Repel More Strongly Than Bonding Pairs!
Lone pair effects on bond angles
In methane there are ________ lone pair electrons so in the tetrahedral shape the bond angles are the expected
109.50.
In the ammonia molecule there is ______________ lone pair. Since it has a stronger repulsion is closes the bond
angle a little to 1070
In the water molecule there are ____________________ lone pairs each having greater repulsion causing the
angle to close even more to 104.50 angle.
Determining Molecular Shape
•
Draw the Lewis Diagram
•
Count up electron pairs on central atom.
double/triple bonds = ONE pair
•
Shape is determined by the # of bonding pairs and lone pairs.
Be Familiar With the Common Shapes & Their Bond Angles (See p.10 in CRM)
Write the bond angle and number of bonding and lone pairs
Common shape-Linear
Common shape-trigonal planar
Common Shape – Bent
Common shape-tetrahedral
Common shape-trigonal pyramidal
Common shape- trigonal bipyramidal
Common shape – octahedral
Molecular Polarity
Dipole Moment
•
Direction of the _____________________ bond in a molecule
•
Arrow points toward the more ___________________________________ atom.
Determining Molecular Polarity
Depends on
Nonpolar Molecules
- dipole moments are _____________________________________ and cancel out
Polar Molecules
- dipole moments are ______________________________________ and don’t cancel
Therefore, polar molecules have…
- asymmetrical shape (lone pairs) or
- asymmetrical atom