NCERT Class XII Chemistry
Chapter - The Solid State
Questions with Solutions
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Question : 7
Total: 50
How will you distinguish between the following pairs of terms ?
(i) Hexagonal close-packing and cubic close-packing
(ii) Crystal lattice and unit cells
(iii) Tetrahedral void and octahedral void
(i) Hexagonal close-packing and cubic close-packing
(ii) Crystal lattice and unit cells
(iii) Tetrahedral void and octahedral void
Solution:
(i) Hexagonal close-packing (hcp) : The first layer is formed utilizing maximum space, thus wasting minimum space. In every second row, the particles occupy the depressions (also called voids) between the particles of the row (fig.). In the thirdrow, the particles are vertically aligned with those in the first row giving AB AB AB....... arrangement. This structure has hexagonal symmetry and is known as hexagonal close-packing (hcp) structure. This packing is more efficient and leaves small spacewhich is unoccupied by spheres. In two dimension central sphere is in contact with six other spheres. Only 26% space is free. In three dimension, the coordination number is 12. A single unit cell has 4 atoms.
Cubic close-packing (ccp) : Again,if we start with hexagonal layer of spheres and second layer of spheres is arranged by placing the spheres over the voids of the first layer, half of these holes can be filled by these spheres. Presume that spheres in the third layer are arranged to coveroctahedral holes. This arrangement leaves third layer not resembling with either first or second layer, but fourth layer is similar to first, fifth layer to second, sixth to third and so on giving pattern ABC ABC ABC....... This arrangement has cubicsymmetry and is known as cubic close-packed (ccp) arrangement. This is also called face centred cubic (fcc).
(ii) Crystal lattice : A regular arrangement of the constituent particles (i.e., atoms, ions or molecules) of a crystal in three dimensionalspace is called crystal lattice or space lattice.
Unit cells : The smallest three dimensional portion of a complete space lattice which when repeated over and over again in different directions produces the complete space lattice is calledthe unit cell.
(iii) The empty spaces left between closed packed spheres are called voids or holes. (
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Cubic close-packing (ccp) : Again,if we start with hexagonal layer of spheres and second layer of spheres is arranged by placing the spheres over the voids of the first layer, half of these holes can be filled by these spheres. Presume that spheres in the third layer are arranged to coveroctahedral holes. This arrangement leaves third layer not resembling with either first or second layer, but fourth layer is similar to first, fifth layer to second, sixth to third and so on giving pattern ABC ABC ABC....... This arrangement has cubicsymmetry and is known as cubic close-packed (ccp) arrangement. This is also called face centred cubic (fcc).
(ii) Crystal lattice : A regular arrangement of the constituent particles (i.e., atoms, ions or molecules) of a crystal in three dimensionalspace is called crystal lattice or space lattice.
Unit cells : The smallest three dimensional portion of a complete space lattice which when repeated over and over again in different directions produces the complete space lattice is calledthe unit cell.
(iii) The empty spaces left between closed packed spheres are called voids or holes.
(a) Octahedral voids : This void is surrounded by six spheres and formed by a combination of two triangular voids of the first and second layer. There is one octahedral void per atom in a crystal.The radius ratio
(b) Tetrahedral voids : These voids are surrounded by four spheres which lie at the vertices of a regular tetrahedron. There are two tetrahedral voids per atom in a crystal and the radius ratio
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