MOLECULAR GEOMETRY TYPES

MOLECULAR GEOMETRY

ORGANIC MOLEC. GEOMETRY  Elmhurst College
Trigonal Bipyramid    Chemistry Department
Octahedral  Virtual ChemBook

 Trigonal Bipyramid Molecular Geometry

Phosphorus Pentachloride:

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An example of trigonal bipyramid molecular geometry that results from five electron pair geometry is PCl5. The phosphorus has 5 valence electrons and thus needs 3 more electrons to complete its octet. However this is an example where five chlorine atoms present and the octet is expanded.

The Lewis diagram is as follows:
Cl = 7 e- x 5 = 35 e-
P = 5 e- = 5 e-

Total = 40 e-

The Chlorine atoms are as far apart as possible at nearly 90o and 120obond angle. This is trigonal bipyramid geometry.

Trigonal bipyramid geometry is characterized by 5 electron pairs.

   

 Sulfur Tetrafluoride:

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In this example, SF4, the Lewis diagram shows S at the center with one lone electron pair and four fluoride atoms attached.

The Lewis diagram is as follows:
S = 6 e-
F = 7 e- x 4 = 28 e-

Total electrons = 34 e-

With four atoms and one lone pair, the electron pair geometry is trigonal bipyramid. The molecular geometry is called see-saw.

Compare this with methane, CH4, which also has four atoms attached but no lone pair.

Notice how the bond angles have been changed considerably from the ideal angles due to the influence of the lone pair repulsion. The axis is bent and the planar atoms are compressed to 102 from the 120 degree ideal angle.

   

Chlorine Trifluoride:

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In this example, ClF3, the Lewis diagram shows chlorine at the center with three fluorine atoms attached and two lone electron pairs.

The Lewis diagram is as follows:
Cl = 7 e-
F = 7 e- x 3 = 21 e-

Total electrons = 28

All fluorine atoms have an octet of electrons, and chlorine has an expanded octet.

The electron pair geometry is trigonal bipyramid and the molecular geometry is T-shape. Again the axial atoms are bent slight from the 180 degree angle.

Compare this with BH3 , which also has three atoms attached but no lone pair. Or ammonia, NH3, with three atoms attached and one lone pair.

 

 Triiodie Ion:

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In this example, I3-, the Lewis diagram shows I at the center with 3 lone electron pair and two other iodide atoms attached.

The Lewis diagram is as follows:

I = 7 e- x 3 = 21 e-

-1 charge = 1 e-

Total electrons = 22 e-

With two atoms attached and three lone pair, the electron pair geometry is trigonal bipyramid. The molecular geometry is called linear.

The triiodide ion is responsible for the blue-black color with starch. Iodine the element alone will not give the color. Iodine in mixture with iodide ions makes the triiodide ion.