VSEPR Theory: Molecular Shapes and Electron Pair Repulsion

What does VSEPR stand for? What does this theory do?

Valence Shell Electron Pair Repulsion

Helps predict molecule shapes

Since covalent bonds have electron pairs with bonds that will

Repel other bonds;

Are all bonds equally spaced out?

No- sometimes:

bonds are closer together so repulsive forces are greater

bonds are further apart so repulsive forces are weaker

All bonds = equally spaced out as far as possible

Why important to determine shape of molecules?

Determine macroscopic properties (melting points,etc)

Predicting how a molecule can react with another

What are the 2 factors that determines the shape of a molecule?

The number of areas of electron density around the central atom

Number of lone pairs and bonding pairs of the electrons on the central atom

Rank these from most to least repulsion:

Lone pair to lone pair

Lone pair to bonding pair

Bonding pair to bonding pair

How to answer shape 5 marker:

1. State the number of areas of electron density around the central atom

2. State the number of lone pairs and number of bonding pairs on the central atom

3. STATE: "Electrons repel to be as far apart as possible and lone pairs repel more than bonding pairs"

4. State the name of the shape

5. State the bond angle of the shape

Warnings about understanding shape models

Single, double, triple and dative bonds bonds still only count as 1 bonding pair in this model

lone pairs on the central atom

Representing the bonds of the molecules

comes forwards out of the plane of the page (so towards you)

goes backwards out of the plan of the page (so away from you)

is on the plane of the page

How many areas of electron density are present in a linear shape?

2 areas of electron density.

What is the bond angle in a linear molecular shape?

180 degrees.

What molecular shape has 3 bonding pairs and 1 lone pair?

Trigonal Pyramidal.

What is the bond angle in a tetrahedral molecular shape?

109.5 degrees.

What shape is formed with 3 bonding pairs and 0 lone pairs?

Trigonal Planar.

What is the bond angle for a trigonal planar shape?

120 degrees.

What is the effect of lone pairs on bond angles?

Lone pairs repel more than bonding pairs, decreasing bond angles.

What is the molecular shape with 5 areas of electron density?

Trigonal Bipyramidal.

What is the bond angle in a trigonal bipyramidal shape?

120 degrees and 90 degrees.

What is the molecular shape with 4 bonding pairs and 2 lone pairs?

Square Planar.

What is the bond angle in an octahedral shape?

90 degrees.

What happens to the shape of a molecule as the number of lone pairs increases?

The shape remains similar, but bond angles decrease slightly.

What is the limitation of VSEPR theory?

It does not work if there are more than 6 pairs of electrons.

Such as if central atom is transition metal

What is the shape name for a molecule with 2 bonding pairs and 1 lone pair?

Bent or Angular.

What is the bond angle for a bent molecular shape?

Less than 120 degrees.

What is the shape name for a molecule with 5 bonding pairs and 0 lone pairs?

Trigonal Bipyramidal.

What is the shape name for a molecule with 3 bonding pairs and 2 lone pairs?

T-shaped.

What is the bond angle in a T-shaped molecular structure?

Less than 90 degrees.

What is the bond angle for a pyramidal shape with 3 bonding pairs and 1 lone pair?

Approximately 107 degrees.