Crystalline Solids
These are the substances whose constituent particles possess a 4. Clean Cleavage with knife
regular orderly arrangement. They are considered as true solids. For
example, NaCl (rock salt), Sucrose (sugar), Diamond, Quartz, etc. A crystalline solid gives a clean surface after cleaving it with a knife
rather than an irregular breakage.
1. Arrangement of constituents particles Classification of crystalline solids
In a crystalline solid, the particles (ion, molecule or atoms) are They can be classified into different categories depending upon the
arranged in definite geometric pattern in the three dimensional type of constituent particles and the nature of intermolecular forces
network. This is known as long range order. This arrangement between them. Various categories are:
repeats periodically over the entire crystal. Due to this arrangement,
they have short range as well as long range order. Ionic solid
Molecular Solids
Covalent or Network Solids
Metallic Solids
Ionic Solids
In the ionic solids, the constituent particles are anions and cations.
Each participating ion is surrounded by a typical number of opposite
charges. The number of ions that surround a particular ion of the
2. Melting point opposite charge is called the coordination number of the ion.
The crystalline solids have a sharp melting point, so possess definite
heat of fusion. For example, in NaCl crystal, each Na+ ion is surrounded by 6
3. Anisotropy
In a crystalline solid, the properties like electrical conductance, Cl- ions and each Cl- ion is surrounded by 6 Na+ ions. So, the
refractive index, thermal expansion, etc., have different values in coordination number of Na+ and Cl- is 6. Similar in CaF2 crystal, the
different directions. coordination number of Ca+ and F- ions are 8 and 4 respectively.
This type of behavior is called Anisotropy and the substances with Some other examples are CaCl, ZnS etc. These ions are held
this property are called Anisotropic. together with strong electrostatic force of attraction.
Some common characteristics of ionic solids are given below.
They have high melting point and boiling point due to strong
electrostatic force of attraction.
In the solid state, they behave like an electrical insulator as
the ions are not free to move, but in an aqueous solution,
they are good conductors of electricity because the ions
become free.
They are generally soluble in a polar solvent but not in a
non-polar solvent. 2. Polar Molecular Solids - These are those solids in which
They are hard and brittle because their stability depends constituent particles are polar molecules like HCl, SO2, etc., the force
upon the preservation of their geometric pattern. which held them together is dipole-dipole force of attraction. This
force is stronger than the dispersion force of non polar molecular
solids.
Molecular Solids
These solids are soft, non conductors of electricity and exist in
In these solids the particles are molecules. Depending upon the gaseous or liquid form at room temperature and pressure. Their
nature of the molecules they are further divided into three types. melting and boiling point is higher than non-polar molecular solids
but not so much.
1. Non-Polar Molecular Solids -They possess the particles either
atoms like those of noble gases or non-polar molecules like H2, 3. Hydrogen-bonded molecular solids - In these types of solids,
Cl2I2 etc. The attractive force between them is weak van der Waal the constituent particles are bonded with hydrogen bond. For
force or dispersion force. example, H2O, NH3, HF, etc.
Due to this weak force, they have low melting and boiling points, are So they possess the strong hydrogen bond as intermolecular force.
soft in nature and non-conductors of electricity (no ions are present). Due to this, they have a high melting and boiling point which are
They are generally gaseous or liquid in nature at room temperature higher than the other two types of molecular solids. They exist as
and pressure. volatile liquid or soft solids at room temperature and pressure.
Covalent or Network Solids
electrons can easily move throughout the whole crystal and form
In this type of solids the constituent particles are atoms of the same
or different elements connected to each other by covalent bond.
So a network of covalent bonds is formed throughout the crystal and the sea of free electrons. So the attractive force between the
they form a giant molecule.
kernels and mobile valence electrons is termed as metallic bond.
The most common example is diamond, silicon carbide (SiC) and
This is the force which holds the metal ions together.
graphite, etc.
Because of these strong metallic bonds, metals can maintain a
regular structure and usually have high melting and boiling points.
Due to strong covalent bonds, they are hard and brittle and have They possess high electrical and thermal conductivity because the
high melting and boiling points. They are insulators but graphite is free electrons flow with a charge or heat energy through the metal.
exception in this case because of the free fourth electron of each
carbon atom where each carbon atom is linked to three neighboring The free electrons allow metal atoms to slide over each other and
carbon atoms. So graphite is good conductor of electricity. can be easily deformed, so metals are malleable and ductile. They
possess bright luster and color due to free electrons. All the metals
The distance between the adjacent layers is greater than carbon- and alloys come in this category.
carbon bond length. These layers are not bonded to each other and Amorphous Solid
can easily slip over each other. This is the reason for the softness These are the substances whose constituent particles don't possess
and good solid lubricant behavior of graphite. a regular orderly arrangement. The main properties of these solids
are mentioned below:
Metallic Solids
1. Arrangement of constituents particles
These solids contain metal atoms as constituent particles. As
In an amorphous solid, the particles are arranged in a regular
manner up to a small region only. This is called short range order.
metals have a good tendency to lose their valence electron and So, in these solids, the particles are not in regular arrangement and
change in to positively charged metal ions (kernel). These
possess only short range order and have irregular shape. They are not considered true solids. Due to short range order, small
parts of amorphous solids may be crystalline and rest may be non-
crystalline. This part of amorphous solids is called crystallizes.
So they are also known as pseudo solids. This is the reason for the
milkiness of window glass of old buildings because due to heating
during the day and cooling at nights, glass acquires some crystalline
character.
Uses of amorphous solids
2. Melting point
The amorphous solids melt gradually over a temperature range, so Amorphous solids are very useful materials. Here are a few
do not possess definite heat of fusion. applications of it:
3. Isotropy
Amorphous solids glass and plastics are very useful
In amorphous solids, the various physical properties like electrical
materials and are widely used in construction, house ware,
conductivity, refractive index, thermal expansion, etc., are same in all
laboratory ware, etc.
directions just like liquids and gases. This type of behavior is called
isotropy and the substances with this property are called isotropic.
So they are isotropic in nature due to random arrangement of Amorphous silica is one of the best materials for converting
particles. sunlight into electricity (photovoltaic).
4. Clean Cleavage with knife Amorphous solid rubber is used in making tyres, shoe soles
etc.
Like a crystalline solid they don't give a clean surface after cleavage Comparison between Crystalline Solids and Amorphous Solids
with knife.
They undergo an irregular breakage.
Crystalline solids Amorphous solids
5. Super cooled liquid Crystals have definite and regular The particles in the constituent are
geometry and have long range as arranged irregularly. They do not
They have a tendency to flow like liquid, although very slowly, so well as short range order of have any definite geometry and
they are also called super cooled liquids. This tendency is illustrated constituent particles. have short range order.
by the fact that glass panes in the window of old historical buildings Crystals posses high melting They are devoid of sharp melting
are found to be thicker at the bottom than at the top. points. points.
6. Pseudo solids
No external regularity in their form
The crystals external forms have
when these amorphous solids are
regularity when these are formed.
formed.
They give a clean surface after Usually the amorphous solids
cleavage with knife. exhibit irregular cut.
Amorphous solids do not possess
They have definite heat of fusion.
any particular heat of fusion.
Amorphous solid do not exhibit
Crystalline solids are very rigid
rigidity. Deformation could be
and their molecules cannot be
done by bending or compressing
deformed by mild distorting force.
them.
Amorphous solids are considered
Crystalline solids are considered
as super cooled liquids or also
as true solid.
pseudo solids.
Crystalline solids display Amorphous solids display
anisotropism. isotropism.
Amorphous graphit
Amorphous Coal
Amorphous coke
Amorphous glass
Amorphous plastic
Amorphous rubber
Crystalline sugar
Crystalline diamond
