The ionic radii of Ma^+ and Xa^- of M_aX_a ionic compounds are 0.102 nm and 0.18

Question

The ionic radii of Ma^+ and Xa^- of M_aX_a ionic compounds are 0.102 nm and 0.181 nm, respectively. The ionic radii of Mb^+ and Xb^- of M_bX_b ionic compounds are 0.061 nm and 0.175 nm, respectively. A student claimed that M_aX_a 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

ANSWER:

The problem can be solved on the bases of radius ratio rule. As per this rule if radius ratio ( cation radius/anion radius) lies between 0.225-0414 tetrahedral geometry will be favoured. If radius ratio is above 0.414 then octahedral structure will be favoured.

Radius ratio of MaXa = radius of cation/radius of anion = 0.102/0.181 = 0.563. Hence tetrahedral configuration will not be possible. This compound will have octahedral confguration.

Radius ratio of MbXb = 0.061/ 0.175 = 0.348. Hence as per radius ratio rule the compound will be present in tetrahedral configuration not in tetrahedral configuration.

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