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The Entropy Reduction Laboratory

Platonic Molecules - ball and stick models
What are
Among the various forces acting between the atoms of a molecule the largest one are those responsible for the ionic and covalent binding. These are the major driving force in determining the molecular structure. Molecules with the symmetry of the Platonic bodies turn out to be of major importance. These we call the Platonic molecules. Ball and stick models are shown in this page for the tetrahedral, octahedral and hexahedral coordination and in the subsequent page for the other two symmetries. The symmetries of the Platonic bodies are realized by two different types of molecules. They are either of a cage structure or a center structure.
the models
The models are shown in maximal size and might not fit into the window. Decrease the size of the models by dragging the mouse, right mousekey pressed. Rotate a selected model by dragging the mouse, left mousekey pressed.
The cage
The tetrahedral symmetry is realized both in the cage and the center structure. Here a molecule of the cage structure is shown. - In the white phosphorus molecule P4 each of the 4 atoms occupies one of the 4 corners of the tetrahedron and is bonded by a covalent bond to the 3 other atoms. The angle between bonds is about 60 degres, which is rather small and results in the instabilty of white phosphorus. Under the influence of light it transforms slowly into the red modification.
The center
In the center mode two kind of atoms are involved. A center atom A (red ball) is bound to 4 atoms B (yellow balls) at the corners of the tetrahedron. The gray lines do not represent chemical bonds, they just ouline the geometry. - This is the most common coordination found in chemistry. A typical example is the methan molecule CH4. It is the most simple of the organic compounds. - Another important example is Freon, CF2Cl2. High up in the atmosphere it is destroyed by ultraviolet light. Free chlorine atoms are formed which react with ozone, O3 molecules, and destroy them.

Apart from the tetrahedral coordination, the coordination 6 is the most common coordination. The central atom A is connected to 6 atoms B at the corner of the tetrahedron. A typical example is sulphurhexafluorid SF6.

The rigid hexahedral coordination is realized only in the CsCl crystal structure. Naturally, in the crystal the atoms B as well as the atoms A appear as center atoms with respect to the surrounding atoms. In the crystal, the number of atoms A is equal to the number of atoms B.
In the realization as an isolated molecule, the hexahedron (cube) twists to form the square antiprism. The center atom A is connected to 8 complexes B at the corners of the twisted structure. This results in less strain on the ligands. Even though each corner lies on the same sphere as in the in case of the untwisted hexahedron, the distance between the repulsive complexes B is increased and thus favoured. Still, the coordination number 8 is not very common. A typical example is the complex [Mo(CN)8]4 -.
Continuation Go to next page for discussion of the dodecahedron and ikosahedron.
(could not be kept on this page for technical reasons)
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Last update of the page: April 20, 2003