Structures with Small Cation-to-Anion Ratios

6 Structures with Small Cation-to-Anion Ratios 

For r -/ra 0.15, the maximum coordination number is 2. Because of the small size of the H ion, substances that are hydrogen bonded have a coordination number of 2 and can only form linear chains unless the molecules involved are multifimctional such as in the case of H2O, which can form a variety of crystalline ices by hydrogen bonding. 

The reader should be cautioned that the radius rules above are by no means absolute and in fact are only obeyed 50% of the time. More ionic compoimds have the rock salt (NaCl) structure and fewer have the CsCl structure than would be predicted from the cation/anion radius. StiU, the use of the radius ratio is a good place to start in trying to imderstand the crystalline structure of binary ionically bonded compoimds. 

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