Question

Which of the following crystals has the unit cell such that a=b≠c and α=β=90°,γ=120° ?

• A. Zinc blende
• B. Graphite
• C. Cinnabar
• D. Potassium dichromate

Answer 1

The Correct Option is B

• The unit cell described in the question where a = b ≠ c, α = β = 90°, and γ = 120° corresponds to the hexagonal crystal system. This is characteristic of graphite, where the carbon atoms are arranged in hexagonal layers.
• Graphite has a hexagonal unit cell, with layers of carbon atoms arranged in a 2D honeycomb structure, and the angle between the axes is 120° for γ. This geometry fits the given unit cell characteristics.

Thus, the correct answer is (B) Graphite.

Answer 2

The structure described corresponds to Graphite. In graphite, the unit cell is hexagonal with the parameters \(a=b=c\) and \(\gamma = 120^\circ\), meaning it forms a hexagonal crystal system.

• Zinc blende has a tetrahedral crystal system.
• Cinnabar has a rhombohedral structure.
• Potassium dichromate crystallizes in an orthorhombic system.