The shapes of molecules is the title of Section 3.3 in Chemical Ideas and it covers the topic of molecular geometry.
Introductory examplesEdit
PolyhedraEdit
TetrahedraEdit
You have probably come across tetrahedra before in maths, although you most likely called them trianglebased 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 metalbased molecules are octahedral.
Common molecular geometriesEdit
Futher examplesEdit
Molecular geometry and lone pairsEdit
You can use the socalled 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 AX_{2}E_{3}. 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 

AX_{1}E_{0} 


Linear  H_{2}  
AX_{1}E_{1} 


Linear  CN^{−}  
AX_{1}E_{2} 


Linear  O_{2}  
AX_{1}E_{3} 


Linear  HCl  
AX_{2}E_{0} 


Linear  BeCl_{2} HgCl_{2} CO_{2} 

AX_{2}E_{1} 


Bent  NO_{2}^{−} SO_{2} O_{3} 

AX_{2}E_{2} 


Bent  H_{2}O H_{2}S OF_{2} 

AX_{2}E_{3} 


Linear  XeF_{2} I_{3}^{−} 

AX_{3}E_{0} 


Trigonal planar  BF_{3} CO_{3}^{2−} NO_{3}^{−} SO_{3} 

AX_{3}E_{1} 


Trigonal pyramidal  NH_{3} PCl_{3} 

AX_{3}E_{2} 


Tshaped  ClF_{3} BrF_{3} 

AX_{4}E_{0} 


Tetrahedral  CH_{4} NH_{4}^{+} PO_{4}^{3−} SO_{4}^{2−} ClO_{4}^{−} 

AX_{4}E_{1} 


Seesaw  SF_{4}  
AX_{4}E_{2} 


Square Planar  XeF_{4}  
AX_{5}E_{0} 


Trigonal Bipyramidal  PCl_{5}  
AX_{5}E_{1} 


Square pyramidal  ClF_{5} BrF_{5} 

AX_{5}E_{2} 


Pentagonal planar  XeF_{5}^{}  
AX_{6}E_{0} 


Octahedral  SF_{6}  
AX_{6}E_{1} 


Pentagonal pyramidal  IF_{6}^{}  
AX_{7}E_{0} 


Pentagonal bipyramidal  IF_{7}  
AX_{8}E_{0} 


Square antiprismatic  IF_{8}^{}  
AX_{8}E_{1} 


Distorted square antiprismatic  XeF_{8}^{2}  
AX_{9}E_{0} 


Tricapped trigonal prismatic OR capped square antiprismatic  ReH_{9}^{2} (tricapped trigonal prismatic) 