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| Radius Ratio Rules |
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| For the stability of an ionic compound, each cation should be surrounded by maximum number of anions and vice versa. The number of oppositely charged ions surrounding each ion is called its co-ordination number. |
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| The larger is the size of the cation the greater is its coordination number. In other wards, greater is the |
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| the greater is its coordination number. |
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| The relationship between the radius and the coordination number and the structural arrangement are called radius ratio rules. |
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| The radius ratio rule is given in the table below. |
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| The above relation may be understood with the following explanation. |
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| For a most stable arrangement the anions must touch each other as well as the cation simultaneously. |
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| For e.g., in a planar triangular structure, the ideal arrangement is represented in the figure. |
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| fig 2.14 |
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| But if the size of the cation increases (Keeping the size of anions same) then the anions will no longer be touching each others as shown in the figure. |
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| fig 2.15 |
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| So, readjust each other in such a way that they touch each other as well as the cation. Under such situation four anions may be touching the cation. |
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| fig 2.16 |
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| The leads to a tetrahedral arrangement. |
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| Similarly in case of an octahedral arrangement, if the size of the cation decreases i.e., the radius ratio decreases the arrangement changes to tetrahedral to become stable. But if the size of the cation increases i.e, the radius ration increases the arrangement changes to body centred cubic structure to acquire stability. |
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| fig 2.17 |
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