`Polar flattening': non-spherical effective shapes of atoms in crystals

An examination of the nearest non-bonded interatomic distances found in crystals shows that where the two atoms are each covalently bound to only one other atom they exhibit non-spherical effective shapes. Such atoms behave as if flattened at their poles. Despite the large number of crystal structures now known in detail, various factors can militate against discovering effective atomic shapes. Possible causes of flattening are briefly discussed.