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When predicting coordination number using radius ratios, several caveats must be kept in mind.

1 The ionic radius and coordination number are not independent. As illustrated by ssss1, effective ionic radius increases as coordination number increases.

2 Since bonds are never truly ionic, models based on spheres in contact are only approximations. As bonds become more covalent and more highly polarized, radius ratios become increasing less effective in predicting coordination numbers.

3 Radius ratios do not successfully predict coordination numbers for metallically bonded substances.

The great value of the concept of coordination polyhedra is that it yields insights into the fundamental patterns in which atoms bond during the formation of crystalline materials. These patterns most commonly involve threefold (triangular), fourfold (tetrahedral), sixfold (octahedral), eightfold (cubic) and, to a lesser extent, 12‐fold coordination polyhedra or small variations of such basic patterns. Other coordination numbers and polyhedron types exist, but are rare in inorganic Earth materials.