SOLID STATE
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Types of Solids
Crystalline solids Amorphous solids
They are true solids.
They are pseudo solids.
They have sharp melting points. They do not have sharp melting points
They are anisotropic in nature. They are isotropic in nature.
They do not have definite heat of
They have definite heat of fusion.
fusion.
They have definite
They have
characteristic geometrical
shape irregular shape
Eg=Salt Eg=Rubber
Isomorphism and polymorphism
Similarity or dissimilarity in crystal
structure of different solids is described
as isomorphism and polymorphism.
ISOMORPHISM
Two or more substances having the same
crystal structure are said to be isomorphous.
In these substances the chemical
composition has the same atomic ratio.
Basic Knowledge(#do not write in EXAM)
Atomic ratio is the simple ratio of the number of
atoms of each element in a chemical compound.
For example (i) NaF and MgO (ii) NaNO3 and
CaCO3 are isomorphous pairs, and have the
same atomic ratios, 1:1 and 1:1:3,
respectively, of the constituent atoms.
Atomic Ratio Atomic Ratio
of NaF of MgO
Mg(1) O(1)
Na(1) F(1)
Atomic Ratio of NaN03 Atomic Ratio of CaCO3
Na (O3) Ca 03
(NO) C
POLYMORPHISM
A single substance that exists in two or more
forms or crystalline structures is said to be
polymorphous.
Examples
1: Calcite and aragonite are two forms of calcium carbonate;
2:α-quartz, b-quartz and cristobalite are three of the several
forms of silica.
Polymorphism occuring in elements is called allotropy.
3 allotropic
of carbon
Diamond Graphite fullerenes
classification of crystalline solids
CRYSTAL STRUCTURE
The ordered three dimensional arrangement of particles in a
crystal is described using two terms, namely, lattice and basis
Lattice is a geometrical arrangement of points in a
three dimensional periodic array.
UNIT CELL
The smallest repeating structural unit of a
crystalline solid is called unit cell.
Types of unit cell
1). Primitive or simple unit cell : In primitive unit cell, the
constituent particles are present at its corners only.
2) Body-centred unit cell : In this type of unit cell, one constituent
particle is present at the centre of its body in addition to the corner
particles.
3) Face-centred unit cell : This unit cell consists of particles at the
centre of each of the faces in addition to the corner particles.
Crystal systems
These 14 lattices, which describe the crystal structure, are called Bravais lattices.
Fourteen Bravais lattices are divided into seven crystal systems.
The seven crystal systems are named as cubic, tetragonal, orthorhombic,
rhombohedral, monoclinic, triclinic and hexagonal system.
Cubic systems
1) Simple cubic unit cell (sc) has a particle at each of the eight corners of a cube.
2) Body-centred cubic unit cell (bcc) has particles at its eight corners and an
additional particle in the center of the cube.
3) Face-centred cubic unit cell (fcc) has particle at the centre of each of six faces
in addition to the particles at eight corners of the cube.
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Number of particles in cubic unit cells
1) Primitive or simple cubic unit cell (sc) :
The corner particle contributes its 1/8th
part to the given unit cell. Thus, a simple
cubic cell has 1/8 × 8 = 1 particle per unit
cell.
2) Body-centred cubic unit cell (bcc)
bcc unit cell has one particle from eight
corners plus one particle in the centre of
the cube, making total of 2 particles per
bcc unit cell.
3) Face-centred cubic unit cell (fcc)
Each particle at the centre of the six faces
is shared with one neighbouring cube.
Thus,1/2 of each face particle belongs to
the given unit cell. From six faces, 1/2 × 6 =
3 particles belong to the given unit cell.
Remember...
Each corner particle of a cube is shared by 8
cubes, each face particle is shared by 2 cubes
and each edge particle is shared by 4 cubes
Relationship between molar mass, . density of the substance and
unit cell edge length, is deduced in the following steps
Edge length of cubic unit cell=a
Volume of unit cell=a3
mass of one particle=m
Particles per unit cell=n
mass of unit cell=m*n....(1)
density of unit cell=ρ
mass of unit cell m*n
ρ=
volume of unit cell = a3
....(2)
M=molar mass of substance
M=mass of particles*number of particles per
mole
M=m*Na........(Na is avogadro number)
M .....(3)
m=
Na
From equation (1) and (3)
M
n M n
ρ= Na
a3
= Na a3
