Crystalline and amorphous solid chemistry
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Solids are characterized by their definite shape and also their considerable mechanical strength and rigidity.
The rigidity due to the absence of translatory motion of the structural units (atoms, ions, etc) of the solids. This web page provides a short description of crystalline and amorphous solids.
In solid chemistry, there are very strong forces of attraction amongst the molecules or ions. This strong forces of attraction due to the absence of translatory motion. But Solid has only the vibrational motion about their mean position.
If external energy supplied to solids the molecules gain translatory energy along with vibrational energy. It transfer solid to liquid.
What are the two types of solids?
The effect of heating is to impart sufficient energy to molecules so that they can overcome these strong forces of attraction. Thus solids are less compressible than liquids and denser than the liquid.
Solids are generally classified into two broad categories
- Crystalline solids
- Amorphous solids
What is the definition of crystalline solid?
The definition of crystalline solids is the solid which posses a definite structure, sharp melting point, and the constituents may be atoms, ions, molecules have order arrangement of the constituents extends in long-range order called crystalline solids.
Examples of crystalline solids
Sodium chloride, potassium chloride, sugar, and ice, quartz are examples of crystalline solids possess a sharp melting point.
The pattern of such crystal having observed in some small crystal region to predict accurately the position of the particle in any region under observation.
Properties of crystalline solids
- In the crystalline, the constituents may be atoms, ions, molecules.
- Crystalline solids have a Sharp melting point, flat faces, and sharp edges. It has a well-developed form and usually arranged symmetrically.
- Definite and the ordered arrangement of the constituents extends over a large distance in the crystal and called the long-range order.
- Crystalline solids those belonging to the cubic class are enantiotropic in nature. The magnitude of the enantiotropic property depends on the direction of measurement.
Amorphous solid definition
The solids which do not possess a definite structure, sharp melting point, and the constituents may be atoms ions, molecules do not have order arrangement of the constituents extends over a short-range the solids called amorphous.
Examples of amorphous solids
Glass, pitch, rubber, plastics, etc are examples of amorphous solids.
Amorphous solids possessing many characteristics of crystalline such as definite shape rigidity and hardness, do not have this ordered arrangement and melt gradually over a range of temperatures.
For this reason, they are not considered as solids but rather highly supercooled liquids.
Difference between crystalline and amorphous solids
Crystalline possess definite structure and sharp melting point but amorphous that do not possess a definite structure and sharp melting point.
Crystalline solids constituents(atoms, molecules) have order arrangement of the constituents extends over a long-range in solids but amorphous the constituents may be atoms, molecules do not have order arrangement.
Classification of crystalline solids
On the basis of the nature of force operating between constituent particles or atoms, ions, molecules of matter, crystalline solids are classified into four categories.
Forces that hold the constituents of molecular crystals are of Van der Waals types. These are weaker forces because of which molecular crystals are soft and possess low melting points.
Carbon dioxide, carbon tetrachloride, argon, and most of the organic compounds are examples of these types of crystals.
The constituent particles of these types of crystalline solids are non-directional.
Hydrogen, helium atom or non-polar hydrogen, oxygen, chlorine, carbon dioxide, methane molecules are examples of these types of crystal. The force operating between constituent particles atoms or molecules is a weak London force of attraction.
Polar binding crystals
The polarity of bond shows in these types of crystalline solids. Sulfur dioxide and ammonia are examples where force operating between constituent particles is the dipole-dipole attraction force.
The constituent molecule of these types of crystalline solids is a polar molecule and these molecules are bound to each other by hydrogen bonding. An example of this type of crystal ice.
Ionic crystal structure
The forces involved here are of electrostatic forces of attraction are stronger than the non-directional type. Thus ionic crystals strong and likely to be brittle.
They have little electricity with high melting and boiling point and can not be bent. The melting point of the ionic crystal increases with the decreasing size of the constituent particles.
In ionic crystals, some of the atoms may be held together by covalent bonds to form ions having a definite position and orientation in the crystal lattice. Calcium carbonate is an example of these types of crystalline solids.
Covalent bonding crystal
The forces involved here are chemical nature or covalent bonds extended in three dimensions.
They are strong and consequently, the crystals are strong and hard with high melting points. The diamond molecule, graphite, silicon are examples of these types of crystalline solid.
Metallic crystalline solids
Electrons are held loosely in these types of crystals. Therefore they are good conductors of electricity. Metallic crystalline solids can be bent and are also strong.
Since the forces have non-directional characteristics the arrangement ao atoms frequently correspond to the closet packing of the sphere.
Forms of carbon in nature
Carbon has several crystalline isotropic forms in nature only two of them are common diamond and graphite molecules.
There are four other rare and poorly understood forms of carbon, β-graphite, Lonsdaleite or hexagonal diamond molecule, Chaoite (very rare mineral) and carbon VI.
The last two forms of carbon appear to contain -C≡C-C≡C- and are closer to the diamond in their properties.
Structure of graphite crystal
The various amorphous forms of carbon like carbon black, soot, etc. are all microcrystalline forms of graphite.
Graphite consists of a layer structure in each layer the C-atoms are arranged in hexagonal planner arrangement with SP² hybridized with three sigma bonds to three neighbors and one π-bonds to one neighbor.
The resonance between structures having an alternative mode of π bonding makes all C-C bonds equal, 114.5 pm equal, consistent with a bond order of 1.33.
The π electrons are responsible for the electrical conductivity of graphite. Weak Van der Waals forces are responsible for binding the successive layers of carbon-atoms and the separation of layers 335pm and the layers easily slide over one another.
Structure of the diamond molecule
Each SP3 hybridized carbon is tetrahedrally surrounded by four other carbon atoms with a C-C bond distance of 154 pm in the diamond molecule. These tetrahedral belong to the cubic unit cell.
Natural diamond molecule commonly contains traces of nitrogen or sometimes very rarely through traces of al in the blue diamond molecule.