Crystalline solids

different kinds of crystalline solids
Crystalline solids
    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.
    These properties are due to the existence of very strong forces of attraction amongst the molecules or ions. It is because of these strong forces that the structural units (atoms, ions, etc) of the solids do not possess any translatory motion but can only have the vibrational motion about their mean position.
    Liquids can be obtained by heating solids up to or beyond their melting points. In solids, molecules do not possess any translatory energy but posses only vibrational energy. The forces of attraction amongst them are very strong.
    The effect of heating solids 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: crystals and amorphous substances.

What are Crystalline solids?

    The solids which posses a definite structure, sharp melting point, and the constituents may be atoms ions or molecules has definite or order arrangement of the constituents extends over a large distance on the solids (long-range order) is called crystalline solids.
    NaCl, KCl, Sugar, and Ice, quartz, etc are the examples of crystalline solids possess a sharp melting point. The pattern is such that having observed it in some small region of the crystal, it is possible to predict accurately the position of the particle in any region under observation.

Characteristics of crystalline solids

In the crystalline solid the constituents may be atoms ions or molecule.

  1. Crystalline solids are a sharp melting point, flat faces and sharp edges which is a well-developed form, are usually arranged symmetrically.
  2. Crystalline solids are definite and the ordered arrangement of the constituents extends over a large distance in the crystal and is called the long-range order.
  3. Crystalline solids (other than those belonging to the cubic class), on the other hand, are enantiotropic in nature. In this case, the magnitude of the property depends on the direction along which it is measured.

Amorphous solids

    The solids which do not possess a definite structure, sharp melting point, and the constituents may be atoms ions or molecules do not have definite or order arrangement of the constituents extends over a short distance on the solids is called amorphous solids.
    Amorphous solids such as glass, pitch, rubber, plastics, etc although possessing many characteristics of crystalline solids such as definite shape rigidity and hardness, do not have this ordered arrangement and melt gradually over a range of temperature.
    For this reason, they are not considered as solids but rather highly supercooled liquids.

Difference between crystalline and amorphous solids

  1. Crystalline solids possess definite structure, sharp melting point but amorphous solids that do not possess a definite structure, sharp melting point.
  2. In crystalline solids constituents(atoms ions or molecules) have definite or order arrangement of the constituents extends over a large distance on the solids (long-range order) but amorphous solids the constituents may be atoms ions or molecules do not have definite or order arrangement.

Classification of crystalline solids

    On the basis of nature of force operating between constituent particles(atoms, ions or molecules) of matter, crystalline solids are classified into four categories,

Molecular crystalline solids

    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.
    CO₂, CCl₄, Ar, and most of the organic compounds are examples of these types of crystals.
    This class further classified into three category
  1. Non-polar molecular crystalline solids
    The constituent particles of these types of crystalline solids are non-directional atom(H, He, etc.) or non-polar molecules(H₂, O₂, Cl₂, CO₂, CH₄, etc.). And the force operating between constituent particles(atoms or molecules) is a weak London force of attraction.
  2. Polar molecular crystalline solids
    The constituent particle of this type of crystalline solids is polar molecules(SO₂, NH₃, etc.)  and the force operating between constituent particles is the dipole-dipole attraction force.
  3. H-bonded molecular crystalline solids
    The constituent molecule of these types of crystalline solids is polar molecule and these molecules are bounded each other by hydrogen bonding. Ice is an example of this type of crystal.

Ionic crystalline solids

    The forces involved here are of electrostatic forces of attraction. These are stronger than the non-directional type. Therefore 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. CaCO₃ is an example of these types of crystalline solids.

Covalent crystalline solids

    The forces involved here are chemical nature (covalent bonds) extended in three dimensions. They are strong and consequently, the crystals are strong and hard with high melting points. Diamond, graphite, silicon, etc. 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.

Isotropic crystalline solids

    Carbon has several crystalline isotropic forms only two of them are common diamond and graphite. There are four other rare and poorly understood allotropes, β-graphite, Lonsdaleite or hexagonal diamond, Chaoite (a very rare mineral) and carbon VI.
    The last two forms appear to contain -C≡C-C≡C- and are closer to the diamond in their properties. 

Structure of graphite

    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. Successive layers of C-atoms are held by weak van der Waals forces at the separation of 335pm and can easily slide over one another.

Structure of diamond

    In diamond, each SP³ hybridized carbon is tetrahedrally surrounded by four other carbon atoms with C-C bond distance 154 pm. These tetrahedral belong to the cubic unit cell.
    Natural diamond commonly contains traces of nitrogen or sometimes very rarely through traces of al in blue diamonds.
    An extremely rare Lonsdalete allotrope found in certain meteorites, the tetrahedral units are stacked to form a hexagonal wurtzite types lattice.

Crystalline solids and amorphous solids, classification of crystalline solids, molecular, ionic, covalent, metallic, and Isotropic crystalline solids

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