Crystalline and Amorphous Solids
Crystalline solids like molecular, ionic, covalent crystals, and metallic solid have defined the orderly arrangement of constituents chemical elements like atoms, ions, or molecules but amorphous solid materials have indefinite types of structure. From the definition of crystalline and amorphous solids, carbon forms diamond crystal structure and amorphous solid materials like carbon black. Crystalline and amorphous solid materials are hard and rigid due to the absence of translatory kinetic energy or motion on the structural units.
The learning of the definite geometrical dimensions of crystalline solids is called crystallography. Bragg diffraction experiment in physics or chemistry is very useful for determination and analysis of crystal structure, developed from the very simple relation between wavelengths of the x-ray electromagnetic radiation spectrum line and spacing between the two lattice planes.
Classification of Molecular Solid Material
A solid molecule is characterized by its definite shape, mechanical strength, density, and rigidity rather than liquid and gases. The rigidity presence due to the absence of the translatory motion of the structural unit of the solid. These units are fixed to their mean position with strong forces of attraction between these units. According to geometrical structure and properties, solid are classified mainly into two types in chemistry like crystalline or crystals solids and amorphous solid materials.
Types of Crystalline Solid in Chemistry
A solid which posses a definite structure, sharp melting point, and symmetrical structural arrangement of constituents is defined as a crystalline state of matter. These properties result to form a high degree of internal order that extends in a definite pattern with long-range order in the crystal.
For example, lithium chloride, sodium chloride, potassium chloride, sugar, and ice, quartzes are the crystalline solid material that has to melt by the specific heat and definite geometrical structural arrangement. Based on the nature of force operating between constituent elements or particles (meaning atoms, ions, molecules) the crystalline solids or crystals are classified into four categories.
Molecular Crystals Material
Van der Waals’s types of forces hold the constituents in the molecular or covalent bonding crystalline solid material. The forces are interatomic or intermolecular in this crystal lattice originating from the dipole-dipole attraction. The forces which combine the molecules are weak Van der Waals or London dispersion forces. Molecular crystals are found in carbon dioxide, nitrogen, mica, borax, boric acid, etc, and most of the organic hydrocarbon like olefin. According to the binding molecule, the molecular crystals are mainly three types, non-polar, polar, and hydrogen bonding crystalline soils.
Some of the molecular crystals describe by non-directional structure due to the absence of the dipole moment. Hydrogen, helium, argon, oxygen, chlorine, carbon dioxide, methane molecule, etc are examples of non-polar crystals. The forces operating between constituent molecules also a weak London dispersion force.
Polar Bonding Crystals
The polar binding crystal has polarity in the constituent’s molecules. Sulfur dioxide and ammonia are common examples of this type of crystalline solid. Due to electric polarization the constituent’s elements also binding by low forces of attraction.
Hydrogen atom carries OH-group and some positive charge attracted to electronegative atoms like oxygen and nitrogen by linking is called hydrogen bonding. The common example is seen in the dimer of formic or acetic acid. The hydrogen bond is electrostatic but very weak with bond energy 5 to 6 kcal. The most discussed example of a hydrogen-bonded crystalline solid is ice where an oxygen atom is surrounded by four hydrogen atoms at the corner of the tetrahedron. A list of organic materials like alcohol, carboxylic acid, proteins is also an example of hydrogen-bonded crystalline solids.
Ionic Crystalline Solids
List of crystals like sodium chloride (NaCl), potassium chloride (KCl), calcium carbonate (CaCO3), etc, the structural units are ion and electrostatic forces operating between these ions. Two forces that operate in ionic crystalline solids, electrostatic attraction forces between the opposite ions and the repulsion effect between the nucleus and inner shell electrons. When attractive and repulsive forces balancing by lowering potential energy the ionic crystal is most stable. Therefore, ionic crystalline solids like NaCl, KCl has properties of high melting point, strong structure, and likely to brittle.
Covalent Crystalline Solids
In many crystals, the atoms in the structural units are held together by covalent forces or bonding by pairing electrons of hybridized orbitals to form the giant type molecules. Diamond, germanium, zinc sulfide, silver iodide, silicon carbide are well-known examples of covalent bonding crystalline solids. In diamond, every carbon atom is covalently linking with the other four carbon atoms along the tetrahedron.
Metallic Crystals Materials
Electron held loosely bound in these types of crystals structure in periodic table metal elements. They are good conductors of electric energy and thermal energy. Metallic crystalline solid can be bent and strongly defined by the metal atom mainly by body-centered or face-centered cubic closed packed and hexagonal solid crystals. In closed paced and hexagonal crystalline form, every metal atom is surrounded by 12 other metal atoms by the metallic bond with coordination number twelve.
Common Properties of Crystalline Solids
- In crystalline solids, the constituents may be atoms, ions, molecules.
- Crystalline solids have the properties of sharp melting points, flat faces, and sharp edges.
- It has a well-developed form and is usually arranged symmetrically. Definite and the ordered arrangement of the constituents extends over a large distance in crystalline solids.
- Crystalline solids belonging to the cubic class shows anisotropic properties but the magnitude of the anisotropic characteristics depends on the direction of structure measurement.
Definition of Amorphous Solid Materials
Amorphous materials define as a solid which does not possess a definite structure, sharp melting point, and the constituents do not form an order arrangement. The constituents extend over a short-range, called short-range order. For example, glass, pitch, rubber, plastics, etc has the properties of amorphous solid.
Amorphous solids define many characteristics of crystalline structure such as shape rigidity and hardness but do not order arrangement and melt gradually over a range of the temperature. Therefore, the amorphous solids are considered supercooled liquids rather than solid.
Difference between Crystalline and Amorphous Solids
- Crystalline meaning a definite structure, a sharp melting point but amorphous does not contain a definite structure, sharp melting point.
- The constituents of crystalline material are ordered arrangement which extends over a long-range in solids. But in an amorphous material define the constituents that do not have order arrangement.
Crystalline or Amorphous form of Carbon
Each sp3 hybridized carbon meanings tetrahedrally surrounded by four other carbon atoms with the carbon-carbon chemical bond distance of 154 pm in the diamond crystals. These tetrahedral structures form a cubic crystal unit of the diamond. Carbon has several covalent crystallines and amorphous solid allotropic forms in nature. Carbon black, Diamond crystals, graphite is excellent examples of both crystalline or amorphous solid for learning chemistry. Material like carbon black, soot, etc. are examples of amorphous solid but all of these forms poly-crystalline structure.
Graphite consists of the layer structure in each layer of the carbon atoms with sp2 hybridized energy levels arranged in hexagonal planner arrangement with free pi-electrons. The pi-electrons are responsible for the electrical conductivity of amorphous graphite material. Successive layers of carbon-atoms attached by weak van der Waals forces with separation of layers 335 pm. Four other rare and poorly understood properties of carbon crystal are β-graphite, Lonsdaleite or hexagonal diamond, Chaoite (very rare mineral), and carbon VI. But the last two forms of carbon appear to contain -C≡C-C≡C- and closer to the diamond in their properties. Carbon has properties to form both types of crystallization forms like crystalline solids and amorphous solid materials for our environment.