12th Class Chemistry The Solid State / ठोस प्रावस्था Question Bank Case Based (MCQs) - The Solid State

Directions: (Q. 11 to 15)

Case III: Read the passage given below and answer the following questions from 11 to 15. The adjective, 'crystalline' when applied to solids, implies an ideal crystal in which the structural units, termed as unit cells, are repeated regularly and indefinitely in three dimensions in space. The unit cell, containing at least one molecule, has a definite orientation and shape defined by the translational vectors, a, b and c.

The unit cell therefore has a definite volume, V that contains the atoms and molecules necessary for generating the crystal. Every crystal can be classified as a member of one of the seven possible crystal systems or crystal classes that are defined by the relationships between the individual dimensions, a, b and c of the unit cell and between the individual angles, \[\alpha \], \[\beta \] and \[\gamma \] of the unit cell. The structure of the given crystal may be assigned to one of the 7 crystal systems, to one of the 14 Bravais lattices, and to one of the 230 space groups. These uniquely define the possible ways of arranging atoms in a three-dimensional solid. Based on these observations, seven crystal systems were identified: triclinic, monoclinic, trigonal or rhombohedral, tetragonal, hexagonal, rhombic or orthorhombic and cubic.

The crystal system of a compound with unit cell dimensions, a = 0.387 nm, b = 0.387 nm and c = 0.504 nm and \[\alpha =\beta =90{}^\circ \]and \[\gamma =120{}^\circ \]is

A) cubic

B) hexagonal

C) orthorhombic

D) rhombohedral.

Correct Answer: B

Solution :

For hexagonal crystal system,\[a=b\ne c\]and \[\alpha =\beta =90{}^\circ ,\,\,\gamma =120{}^\circ \]