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:

Hydrogen cyanide (HCN)
Ethyne (C₂H₂)

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 SO₃^2- (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⁺, SO₄^2-, PO₄^3-, 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^-, CO₃^2-, 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^-, CO₃^2-, C₂H₄
Tetrahedral	4	0	109.5	NH₄⁺, SO₄^2-, PO₄^3-, Ni(CO)_4,CH₄
Trigonal pyramidal	3	1	107	PH₃, SO₃^2-, NH₃
Non-linear (Crooked)	2	2	105	H₂S, SO₂, H₂O

Questions – see handwritten sheet.