Orientation around bond, standard

Two important types of polymer chain are the planar zigzag and the helix, but whatever form the chains take in crystallites, they must be straight on a crystallite-size scale and the straight chains must pack side by side parallel to each other in the crystal. The ideas of standard bond lengths, angles and orientations around bonds discussed in chapter 3 can be used to predict likely possible low-energy model chain conformations that exhibit translational symmetry (see example 4.3). As discussed in the... 

By assuming the standard bond lengths, angles and preferred orientations around bonds, estimate the most likely chain repeat length for polyethylene. 

The simplest models of covalently bonded chemical substances are the so-called ball-and-stick models, in which the atoms are represented by balls and the sticks represent the bonds. In constructing such models it is usual to make use of so-called standard bond lengths and standard bond angles . In addition, certain orientations of groups of atoms around bonds are also assumed, i.e. preferred torsional angles . These standards correspond approximately to properties of the real molecules and control the possible shapes that the molecules can take. What are the reasons for these values and what are they for various types of molecule ... 

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