Types of crystalline solids and their properties
What is the crystalline solid?
Solids are characterized by their definite shape and also their considerable mechanical strength and rigidity. The rigidity due to the absence of translational energy 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 molecule or ion in chemistry. 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 with a specific heat 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 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 and amorphous substances.
On the basis of chemical bonding operating between constituent particles (atoms, ions, molecules) of substances, crystalline solids are classified into four categories,
Molecular crystalline solid
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, methane, ice water, and most of the organic hydrocarbon are examples of these types of crystals. This class further classified into three category
1. Non-polar binding molecules
The constituent particles of these types of crystalline solids are non-directional atom(Hydrogen, helium, etc.) or non-polar molecules like hydrogen, oxygen, methane, etc. And the force operating between constituent particles (atoms or molecules) is a weak London force of attraction.
2. Polar binding molecules
The constituent particle of this type of crystalline solids is polar (SO₂, NH₃, etc.) and the polarity of bonds depends on the dipole-dipole attraction between the molecules.
3. Hydrogen-bonded molecule
The constituent molecule of these types of crystalline solids is polar and these molecules are bounded each other by hydrogen bonding. An example of this type of crystal is ice.
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.
Isotopic forms of the carbon atom
Carbon has several crystalline isotopes, only two of them are common diamonds 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.
Amorphous forms of carbon
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 orbitals 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.
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