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.

Introductory examplesEdit

Methane moleculeEdit

Ammonia moleculeEdit

Water moleculeEdit

Hydrogen fluoride moleculeEdit

PolyhedraEdit

TetrahedraEdit

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).

OctahedraEdit

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.

Common molecular geometriesEdit

Futher examplesEdit

Example moleculesEdit

Example ionsEdit

Molecular geometry and lone pairsEdit

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 AX1E0-2D.png AX1E0-2D.png AX1E0-3D-balls.png AX1E0-3D-balls.png H2
AX1E1
1
1
Linear AX1E1-2D.png AX1E0-2D.png AX1E1-3D-balls.png AX1E0-3D-balls.png CN
AX1E2
1
2
Linear AX1E2-2D.png AX1E0-2D.png AX1E2-3D-balls.png AX1E0-3D-balls.png O2
AX1E3
1
3
Linear AX1E3-2D.png AX1E0-2D.png AX1E3-3D-balls.png AX1E0-3D-balls.png HCl
AX2E0
2
0
Linear AX2E0-2D.png AX2E0-2D.png AX2E0-3D-balls.png Linear-3D-balls.png BeCl2
HgCl2
CO2
AX2E1
2
1
Bent AX2E1-2D.png AX2-bent-2D.png AX2E1-3D-balls.png Bent-3D-balls.png NO2
SO2
O3
AX2E2
2
2
Bent AX2E2-2D.png AX2-bent-2D.png AX2E2-3D-balls.png Bent-3D-balls.png H2O
H2S
OF2
AX2E3
2
3
Linear AX2E3-2D.png AX2E0-2D.png AX2E3-3D-balls.png Linear-3D-balls.png XeF2
I3
AX3E0
3
0
Trigonal planar AX3E0-side-2D.png AX3E0-side-2D.png AX3E0-3D-balls.png Trigonal-3D-balls.png BF3
CO32−
NO3
SO3
AX3E1
3
1
Trigonal pyramidal AX3E1-2D.png AX3-pyramidal-2D.png AX3E1-3D-balls.png Pyramidal-3D-balls.png NH3
PCl3
AX3E2
3
2
T-shaped AX3E2-2D.png AX3-T-shaped-2D.png AX3E2-3D-balls.png T-shaped-3D-balls.png ClF3
BrF3
AX4E0
4
0
Tetrahedral AX4E0-2D.png AX4E0-2D.png AX4E0-3D-balls.png Tetrahedral-3D-balls.png CH4
NH4+
PO43−
SO42−
ClO4
AX4E1
4
1
Seesaw AX4E1-2D.png AX4-seesaw-2D.png AX4E1-3D-balls.png Seesaw-3D-balls.png SF4
AX4E2
4
2
Square Planar AX4E2-2D.png AX4-square-planar-2D.png AX4E2-3D-balls.png Square-planar-3D-balls.png XeF4
AX5E0
5
0
Trigonal Bipyramidal AX5E0-2D.png AX5E0-2D.png Trigonal-bipyramidal-3D-balls.png Trigonal-bipyramidal-3D-balls.png PCl5
AX5E1
5
1
Square pyramidal AX5E1-2D.png AX5-square-pyramidal-2D.png AX5E1-3D-balls.png Square-pyramidal-3D-balls.png ClF5
BrF5
AX5E2
5
2
Pentagonal planar AX5E2-2D.png AX5E2-3D-balls.png Pentagonal-planar-3D-balls.png XeF5-
AX6E0
6
0
Octahedral AX6E0-2D.png AX6E0-2D.png AX6E0-3D-balls.png Octahedral-3D-balls.png SF6
AX6E1
6
1
Pentagonal pyramidal AX6E1-2D.png AX6-pentagonal-pyramidal-2D.png AX6E1-3D-balls.png Pentagonal-pyramidal-3D-balls.png IF6-
AX7E0
7
0
Pentagonal bipyramidal AX7E0-2D.png AX7E0-2D.png AX7E0-3D-balls.png Pentagonal-bipyramidal-3D-balls.png IF7
AX8E0
8
0
Square antiprismatic AX8E0-3D-balls.png Square-antiprismatic-3D-balls.png IF8-
AX8E1
8
1
Distorted square antiprismatic Square-antiprismatic-3D-balls.png XeF82-
AX9E0
9
0
Tricapped trigonal prismatic OR capped square antiprismatic AX9E0-3D-balls.png AX9E0-3D-balls.png ReH92- (tricapped trigonal prismatic)
Last modified on 26 March 2013, at 19:13