DIRECTIONS: Read the passage given below and answer the questions that follow :
In hexagonal systems of crystals, a frequently encountered arrangement of atoms is described as a hexagonal prism. Here, the top and bottom of the cell are regular hexagons and three atoms are sandwiched in between them. A space-filling model of this structure, called hexagonal close-packed (hcp), is constituted of a sphereona flat surface surrounded in the same plane by six identical spheres as closely as possible. These spheres are then placed over the first layer so that they touch each other and represent the second layer. Each one of these three spheres touches three spheres of the bottom layer. Finally, the second layer is covered with third layer that is identical to the bottom layer in relative position. Assume radius of every sphere to be ‘r’.28.
The empty space in this hcp unit cell is

DIRECTIONS: Read the passage given below and answer the questions that follow :
In hexagonal systems of crystals, a frequently encountered arrangement of atoms is described as a hexagonal prism. Here, the top and bottom of the cell are regular hexagons and three atoms are sandwiched in between them. A space-filling model of this structure, called hexagonal close-packed (hcp), is constituted of a sphereona flat surface surrounded in the same plane by six identical spheres as closely as possible. These spheres are then placed over the first layer so that they touch each other and represent the second layer. Each one of these three spheres touches three spheres of the bottom layer. Finally, the second layer is covered with third layer that is identical to the bottom layer in relative position. Assume radius of every sphere to be ‘r’.28.

The volume of this hcp unit cell is –

DIRECTIONS: Read the passage given below and answer the questions that follow :
In hexagonal systems of crystals, a frequently encountered arrangement of atoms is described as a hexagonal prism. Here, the top and bottom of the cell are regular hexagons and three atoms are sandwiched in between them. A space-filling model of this structure, called hexagonal close-packed (hcp), is constituted of a sphereona flat surface surrounded in the same plane by six identical spheres as closely as possible. These spheres are then placed over the first layer so that they touch each other and represent the second layer. Each one of these three spheres touches three spheres of the bottom layer. Finally, the second layer is covered with third layer that is identical to the bottom layer in relative position. Assume radius of every sphere to be ‘r’.28.

The number of atoms in the hcp unit cell is

Which of the following exists as covalent crystals in the solid state ?

Lithium forms body centred cubic structure. The length of the side of its unit cell is 351 pm. Atomic radius of the lithium will be :

Percentages of free space in cubic close packed structure and in body centered packed structure are respectively

The Ca²⁺ and F–are located in CaF₂ crystal, respectively at face centred cubic lattice points and in