The ionic radii of Ma+ and XV of M_aX_a ionic compounds are 0.102 11111 and 0.18

Question

The ionic radii of Ma+ and XV of M_aX_a ionic compounds are 0.102 11111 and 0.181 nm, respectively. The ionic radii of M_b+ and M_b+ of M_bX_b ionic compounds are 0.061 nm and 0.175 nm, respectively. A student claimed that MaXa ionic compounds have a tetrahedral configuration with coordination number of 4 and M_bX_b ionic compounds have an octahedral configuration with coordination number of 6. Is this feasible? If so, state why it is feasible. If not, provide your reason why it is not possible.

Explanation / Answer

For MaXa, radius ratio of cation to anion = 0.102/0.181 = 0.564

For MbXb, radius ratio of cation to anion = 0.061/0.175 = 0.349

Note: If radius ratio is between 0.225 and 0.414, then the compound will have a tetrahedral (coordination number 4) configuration. If radius ratio is between 0.414 and 0.732, then the compound will have an octahedral configuration (coordination number 6). If radius ratio is greater than 0.732, then the compound will have a cubic configuration (coordination number 8).

Hence, the students claim is wrong and not feasible. This is because, according to above note, MaXa should have an octahedral configuration while MbXb should have a tetrahedral configuration.

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