which of the following is not true about the ionic solids ?

Options

A. Bigger ions form the close packed structure
B. Smaller ions occcupy either the tetrahedral or the octahedral voids depending upon their size.
C. Occupation of all the voids is not necessary
D. The fraction of octahedral or tetrahedral voids occupied depends upon the radii of the ions occupying the voids.

Explanation

To determine which statement is not true about ionic solids, let’s analyze each option step by step.

### Step 1: Analyze the First Statement
**Statement:** Bigger ions form closed packed structure.

- In ionic solids, the larger ions (anions) do indeed form a close-packed structure. For example, in sodium chloride (NaCl), the chloride ions (Cl⁻) are larger than the sodium ions (Na⁺) and they occupy the close-packed arrangement. Thus, this statement is **true**.

### Step 2: Analyze the Second Statement
**Statement:** Smaller ions occupy either tetrahedral or octahedral voids depending upon their size.

- Smaller ions (cations) can occupy the voids in the close-packed structure of the larger ions. The size of the cation determines whether it will occupy a tetrahedral or octahedral void. Smaller cations fit into tetrahedral voids, while larger cations fit into octahedral voids. Hence, this statement is also **true**.

### Step 3: Analyze the Third Statement
**Statement:** Occupation of all voids is not necessary.

- In ionic solids, it is not necessary for all voids to be occupied by cations. Some voids may remain empty depending on the ratio of cations to anions. This statement is therefore **true**.

### Step 4: Analyze the Fourth Statement
**Statement:** The fraction of octahedral and tetrahedral voids occupied depends upon the radii of ions occupying the voids.

- The fraction of voids occupied does not solely depend on the radii of the ions. While the size of the cation does influence whether it occupies a tetrahedral or octahedral void, the actual fraction of voids occupied is determined by the overall stoichiometry and arrangement of ions in the crystal lattice, not just by their radii. Therefore, this statement is **false**.

### Conclusion
The statement that is **not true** about ionic solids is: **The fraction of octahedral and tetrahedral voids occupied depends upon the radii of ions occupying the voids.**

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