Question

In Chrysoberyl, a compound containing Beryllium, Aluminium and oxygen, oxide ions form cubic close packed structure. Aluminium ions occupy \(\frac{1}{4}\)th of octahedral voids. The formula of the compound is

• A. BeAlO₄
• B. BeAl₂O₄
• C. Be₂AlO₂
• D. BeAlO₂

Answer 1

The Correct Option is B

Step 1: Analyze the CCP structure

In a cubic close-packed structure, the oxide ions (O²⁻) form the close-packed arrangement, which leaves octahedral voids. The number of octahedral voids is equal to the number of oxide ions in the structure.

Step 2: Consider the Aluminium ions

We are told that aluminium ions (Al³⁺) occupy 1/4th of the octahedral voids in the CCP structure. This means that for every 4 oxide ions, there is 1 aluminium ion occupying an octahedral void.

Step 3: Determine the ratio of ions

- The ratio of Al to O is 1:4 because Al occupies 1/4th of the octahedral voids.
- The beryllium ions (Be²⁺) are likely in tetrahedral voids in the structure.

Step 4: Construct the formula

Given that there are 4 oxygen ions (O²⁻) for every aluminium ion (Al³⁺), the number of Be ions will balance the structure. The most likely formula for the compound would be BeAl₂O₄.

Final Answer: The formula of the compound is BeAl₂O₄.

Answer 2

In the given compound, the oxide ions form a cubic close-packed structure. The aluminum ions occupy 1/4th of the tetrahedral voids and the beryllium ions occupy 1/4th of the octahedral voids.

• In a cubic close-packed structure, the number of tetrahedral voids and octahedral voids are both equal to 4.
• Aluminum occupies 1/4th of the tetrahedral voids, so it will occupy 1 tetrahedral void.
• Beryllium occupies 1/4th of the octahedral voids, so it will occupy 1 octahedral void.
• There are 4 oxide ions in the unit cell.

Therefore, the formula of the compound is \(BeAl₂O₄\).

Correct answer is (B): \(BeAl₂O₄\)