The Geometry of Multiple Bonds

Experimental evidence shows that the two or three electron pairs in a multiple bond behave as a single electron pair in establishing molecular geometry. This appears if we compare beryllium difluoride, carbon dioxide, and hydrogen cyanide, whose Lewis diagrams are 

All three molecules are linear their bond angles are 180°. The two electron pairs in BeF2 are as far from each other as possible. According to the VSEPR principle, this is responsible for the 180° bond angle in that compound. Carbon is flanked by two double bonds in CO2 and one single bond and one triple bond in HCN. Evidently, the second and third electron pairs in double and triple bonds don t affect the molecular geometry. 

Further evidence supporting this conclusion comes from comparing the bond angles in boron trifluoride and formaldehyde  

The shapes are both trigonal planar with 120° bond angles. This is the angle predicted for three electron pairs under the VSEPR theory. 

Determine the molecular geometry of ethylene, C2H4. You will need to describe the geometry around each carbon atom. Sketch a three-dimensional ball-and-stick representation of the molecule. 

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Section 13.3 Molecular Geometry Section 13.4 The Geometry of Multiple Bonds... 

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