MOLECULARSTRUCTURE

MOLECULAR GEOMETRY

VSEPR THEORYValence-Shell Electron-Pair Repulsion Theory:

Because electron pairs repel, molecules adjust their shapes so that valence electron pairs are as far apart as possible

VSEPR CONTINUED…

Types of electron pairs

Bonding Pairs: Electrons that form bonds

Lone (unshared) Pairs: Electrons that are nonbonding; no atom trying to bond. These are held closer to the atom they are associated with than bonding pairs.

Lone Pairs Repel More Strongly Than Bonding Pairs!

LONE PAIR EFFECTS ON BOND ANGLESIn methane there are no lone pair electrons so in the tetrahedral shape the bond angles are the expected 109.50.

In the ammonia molecule there is one lone pair. Since it has a stronger repulsion is closes the bond angle a little to 1070

In the water molecule there are 2 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

COMMON SHAPE-LINEARBeryllium Hydride

2 total electron pairs

2 bonding pairs

0 lone pairs

Beryllium does not follow the octet rule. Beryllium is complete with 4 valence electrons

Carbon dioxide

2 total electron pairs

2 bonding pairs

0 lone pairs

Carbon does follow the octet rule fulfilling it through two double bonds in this case

COMMON SHAPE-TRIGONAL PLANARBoron Hydride

3 total electron pairs

3 bonding pairs

0 lone pairs

Boron is also an exception to the octet rule. Boron is complete with 6 valence electrons

Formaldehyde

4 total electron pairs

4 bonding pairs

0 lone pairs

Carbon does follow the octet rule, which is fulfilled by the double bond.

COMMON SHAPE - BENTOzone

Generally O would have two bonding pairs which is seen by the upper O, the other two adjust so that all three can obey the octet rule

Water

4 total pairs

2 bonding pairs

2 lone pair

COMMON SHAPE-TETRAHEDRALMethane

4 total pairs

4 bonding pairs

0 lone pairs

COMMON SHAPE-TRIGONAL PYRAMIDALAmmonia

4 total pairs

3 bonding pairs

1 lone pair

COMMON SHAPE- TRIGONAL BIPYRAMIDALPhosphorous pentachloride

5 total pairs

5 bonding pairs

0 lone pairs

COMMON SHAPE - OCTAHEDRALSulfur Hexafluoride

6 total pairs

6 bonding pairs

0 lone pairs

POLAR MOLECULES

Therefore, polar molecules have…

- asymmetrical shape (lone pairs) or

- asymmetrical atoms