Shapes of Molecules

What do I need to be able to do?

Linear Molecules

Crooked shapes

Trigonal Pyramidal

Tetrahedral

Trigonal Planar

Summary

Some of the images are available in Chime, which lets the viewer see them in 3D and allows manipulation. 

In molecules there are 2 types of electron

1. Bonding Pairs

2. Non-bonding or lone pairs

 

The combinations of these determine the shape of the molecule

 

Important: p - bonds have a very limited impact on shape. s-bonds have a much more important effect.

 

Definition: Shapes of molecules is determined by Electron-Pair Repulsion Theory (EPRT). The outer pairs of electrons around a covalently bonded atom minimise the repulsions between them by moving as far apart as possible.

The standard example is ammonia (NH3) [Chime View]

What do I need to be able to do?

 Predict the shapes and bond angles in molecules and ions by using the Electron-Pair Repulsion Theory for up to 4 electron pairs (including lone pairs).

 The board specifies these as examples:

Molecule

Shape

BF3 (boron trifluoride)

Trigonal

CO2 (carbon dioxide)

Linear

CH4 (methane)and NH4+ (ammonium ion)

Tetrahedral

NH3 (ammonia)

Pyramidal

H2O (water)

Non-linear or bent

The board specifies that you are able to predict the shapes of molecules analogous to these

1.                Linear Molecules.

 These are the simplest shapes, involving only 2 bonding pairs

 The simplest is BeCl2[Chime View]

Carbon dioxide (CO2) is another example. It has bonding pairs which all repell equally.

  

So the bond angle O=C=O is 180°. [View in Chime]

 Other examples include:

2.              Crooked shapes

 The standard example is water, the unique properties of water are closely linked to its shape.

1)                  Only 3 atoms so must be linear

2)                  Bond angle is 104.5°

3)                  Shape impact on water’s strange behaviour

Any molecule with 2 bonding pairs and 2 lone pairs will have a crooked shape, giving a bond angle significantly less than the angle on a tetrahedral structure

[Chime Version] of water here

 

Other examples include H2S and SO2.

 3.                Trigonal Pyramidal

 3 bonding pairs and 1 lone pair = Trigonal pyramidal.

 Eg ammonia

 

 [Chime View] here

Notice the bond angle of 107°.

 Other examples include PH3 (phosphine) and SO32- (sulphite)

 

4.                Tetrahedral

4 bonding pairs all with equal bonds gives a tetrahedral structure with a bond angle of 109.5° in methane.

  [Chime View] of Methane

Other examples include NH4+, SO42-, PO43-, Ni(CO)4

5.                Trigonal planar

These contain 3 – bonding pairs and no non-bonding pairs.

 Boron trifluoride is an example of this (BF3). Bond angle of 120°.

[Chime Version] here

Examples include BF3, SO3, NO3-, CO32-, C2H4.

Summary table

Shape

Bonding-pairs

Non-bonding pairs

Bond angle /°

Examples 

Linear

2

0

180

BeCl2, CO2, HCN, C2H2

Trigonal planar

3

0

120

BF3, SO3, NO3-, CO32-, C2H4

Tetrahedral

4

0

109.5

NH4+, SO42-, PO43-, Ni(CO)4, CH4

Trigonal pyramidal

3

1

107

PH3, SO32-, NH3

Non-linear (Crooked)

2

2

105

H2S, SO2, H2O

Questions – see handwritten sheet.