=> Ans. Polymorphous: A single substance which crystallises in two or
more forms under different conditions of solidification is called.
polymorphous. Polymorphic forms of an element are called allotropic
forms or allotropes. For example, carbon exists as diamond and graphite,
or sulphur exists in rhombic and monoclinic allotropic forms.
2➤
What are the types of particles in each of the four main classes of
crystalline solids?
=> Ans. The smallest constituents or particles of various solids are
atoms, ions or molecules.
3➤ What are molecular crystals?
=> Ans. The crystals in which the constituent particles are molecules
(or unbonded single atoms) of the same substance held. together by
intermolecular forces of attraction. For example, solidified Cl₂, CO₂,
etc.
4➤ What is crystal lattice (space lattice)?
=> Ans. Crystal lattice (space lattice): A regular arrangement of the
constituent particles (atoms, ions or molecules) of a crystalline solid
having similar environment in three dimensional space is called crystal
lattice or space lattice.
5➤ What is a unit cell?
=> (1) Unit cell: It is the smallest repeating structural unit of a
crystalline solid (or crystal lattice) which when repeated in different
directions produces the crystalline solid (lattice). (2) The unit cell
possesses all the characteristics of the crystalline solid.
6➤
Mention two properties that are common to both hcp and ccp
lattices.
=> Ans. In hcp and ccp crystal lattices coordination number is 12 and
packing efficiency is 74%.
7➤
A solid is hard, brittle and electrically non-conductor. Its melt conducts
electricity. What type of solid is it?
=> Ans. A solid crystalline electrolyte like NaCl is hard, brittle and
electrically nonconductor. But its melt conducts electricity
8➤
Write the effect on density of a substance in the Frenkel defect?
=> Ans: In the Frenkel defect, the density of the substance remains
unchanged As no ions are missing from the crystal lattice as a
whole.
9➤
Name the bravais lattice in the triclinic system.
=> Ans: The Bravais lattice in the triclinic system is simple or
primitive unit cell.
10➤ What are diamagnetic substances?
=> Ans: The substances with all electrons paired, are weakly repelled
by magnetic fields. Such substances are called as diamagnetic
substances.
11➤
Give one property common to both hep and cep crystal lattices.
=> Ans: One property common to both hcp and ccp lattice is same
coordination number of the particles (i.e., 12).
12➤
The number of tetrahedral voids are formed if the number of atoms in a
crystal is N/2.
=> Ans: If the number of atoms in a crystal is N/2, the number voids is
N. tetrahedral
13➤
Give the percentage of empty space in bcc lattice.
=> Ans: The percentage of empty space in bcc lattice is 32%.
14➤
If the total volume of a simple cubic unit cell is 6.817 x 10.2 cm³, what
is the volume occupied by particles in unit cell?
=> Ans: Volume occupied by particles in simple cubic unit cell na
3.14×6.817×10 cm = 3.57 x10 cm³ 6 6
15➤
The number of octahedral voids formed in 0.5 mol of a compound forming hep
structure?
=> Ans: Number of particles in 0.5 mol = 0.5 x NA = 0.5 x 6.022 x 1023
=3.011 x 1023 Number of octahedral voids Number of particles - 3.011 x
10²3
