The Sizes and Shapes of Organic Molecules. Molecular Models

2-2 THE SIZES AND SHAPES OF ORGANIC MOLECULES. MOLECULAR MODELS 

The size and shape of molecules are as much a part of molecular structure as is the order in which the component atoms are bonded. Contrary to the impression you may get from structural formulas, complex molecules are not flat and formless, but have well-defined spatial arrangements that are determined by the lengths and directional character of their chemical bonds. It is not easy to visualize the possible arrangements of the bonds in space and it is very helpful to have some kind of mechanical model that reflects the molecular geometry, including at least an approximation to the relative lengths of the bonds. Ball-and-stick models such as the ones used by Patemo (Section 1-ID) fill this purpose admirably. 

Whereas methane, CH4, is tetrahedral, ethene, C2H4, is not. According to the best available physical measurements, all six atoms of ethene lie in a single plane and the H—C—H bond angles are 117.3°. Methanal (formaldehyde) also is a planar molecule with an H—C—H bond angle of 118°. 

Ethyne, C2H2, has been established experimentally to be a linear molecule that is, the H—C—C bond angle is 180°  

2 Structural Organic Chemistry. The Shapes of Molecules. Functional Groups 

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