A-level Chemistry/OCR (Salters)/Molecular geometry

The shapes of molecules is the title of Section 3.3 in Chemical Ideas and it covers the topic of molecular geometry.

Polyhedra

Tetrahedra

You have probably come across tetrahedra before in maths, although you most likely called them triangle-based pyramids. Tetrahedra have four vertices (corners), four faces and six edges. Each face is an equilateral triangle.

The tetrahedron is one of the most important shapes in chemistry because a very great many molecules contain them. Tetrahedral molecules don't actually contain little pyramids. What they do contain is a central atom bonded to four other atoms. The four atoms surrounding the central atom occupy positions that you can imagine as the vertices of a tetrahedron.

In the image gallery below, the central atom is coloured magenta and the surrounding atoms are coloured white.

The angle between any two bonds in a tetrahedral molecule is approximately 109.5°. The tetrahedral angle can be calculated as accurately as required because it is equal to cos−1(–⅓).

Octahedra

You may or may not have met an octahedron before. Octahedra have six vertices (corners), eight faces and twelve edges. Each face is an equilateral triangle.

Octahedra are very important in chemistry because many transition metal-based molecules are octahedral.

Molecular geometry and lone pairs

You can use the so-called AXE method to calculate the shape of a molecule. It is based on molecules that have a central atom, which we label A. Atoms or groups bonded to A are labelled X. Lone pairs are labelled E. A molecule with three lone pairs and two atoms/groups bonded to it would be denoted AX2E3. The table below shows how X and E and molecular shape are related.

Valence shell electron pair repulsion theory (VSEPR) is used to predict the shape of a molecule once X and E are known. This sounds more complicated than it is. You consider any X's and E's to be regions of charge that position themselves as far apart from each other as possible, in order to minimize the forces of electrostatic repulsion between each other.

AXE label X
(substituents)
E
(lone pairs)
Shape 2D diagram
lone pairs shown
2D diagram
lone pairs not shown
3D model
lone pairs shown
3D model
lone pairs not shown
Examples
AX1E0
1
0
Linear H2
AX1E1
1
1
Linear CN
AX1E2
1
2
Linear O2
AX1E3
1
3
Linear HCl
AX2E0
2
0
Linear BeCl2
HgCl2
CO2
AX2E1
2
1
Bent NO2
SO2
O3
AX2E2
2
2
Bent H2O
H2S
OF2
AX2E3
2
3
Linear XeF2
I3
AX3E0
3
0
Trigonal planar BF3
CO32−
NO3
SO3
AX3E1
3
1
Trigonal pyramidal NH3
PCl3
AX3E2
3
2
T-shaped ClF3
BrF3
AX4E0
4
0
Tetrahedral CH4
NH4+
PO43−
SO42−
ClO4
AX4E1
4
1
Seesaw SF4
AX4E2
4
2
Square Planar XeF4
AX5E0
5
0
Trigonal Bipyramidal PCl5
AX5E1
5
1
Square pyramidal ClF5
BrF5
AX5E2
5
2
Pentagonal planar XeF5-
AX6E0
6
0
Octahedral SF6
AX6E1
6
1
Pentagonal pyramidal IF6-
AX7E0
7
0
Pentagonal bipyramidal IF7
AX8E0
8
0
Square antiprismatic IF8-
AX8E1
8
1
Distorted square antiprismatic XeF82-
AX9E0
9
0
Tricapped trigonal prismatic OR capped square antiprismatic ReH92- (tricapped trigonal prismatic)