Crystalline solids and amorphous solids
Crystalline solids and amorphous solids are the two types of solid formed by atoms, ions, or molecules. Molecular, ionic, covalent, and metallic crystalline solid are the main types of crystals formed by their constituents with a definite structure. An amorphous solid is a material that does not possess a definite structure and sharp melting point.
In chemistry, a solid molecule is characterized by its definite shape, strength, density, and rigidity rather than liquid and gases. Solids are rigid due to the absence of translatory motion on the structural unit. These units are fixed to their mean position with strong forces of attraction.
A crystal or crystalline solid is a solid material where constituents like atoms, molecules, or ions are arranged in a definite and orderly arrangement over a large distance in the crystal lattice. This is called long-range order.
Examples of crystalline solid
Lithium chloride, sodium chloride, potassium chloride, sugar, ice, and quartz are the most common examples of crystalline solid. These materials are melted by a specific heat with a definite geometrical structural arrangement.
Structure of crystalline solids
Crystalline solids have well-defined edges and faces with definite melting points. The study of the geometrical structure in the crystal lattice is called crystallography.
The structure of the sodium chloride crystal is given below the picture,
Bragg diffraction experiment in physics or chemistry is very useful for the determination and analysis of crystal structure. It is developed from the very simple relation between wavelengths of the x-ray radiation and spacing between the two lattice planes.
Properties of crystalline solids
- The constituents in crystalline solids may be atoms, ions, molecules.
- Crystalline solids have the properties of sharp melting points, flat faces, and sharp edges.
- It is a well-developed form that is arranged symmetrically. Definite and the ordered arrangement of the constituents extends over a large distance in crystal lattices.
- Crystalline solids belonging to the cubic class shows anisotropic properties. The magnitude of the anisotropic characteristics depends on the direction of structure measurement.
Amorphous solid is a material that does not possess a definite structure, sharp melting point, and the constituents do not form an order arrangement. The constituents are extending over a short distance. This is called short-range order.
Examples of amorphous solids
Glass, pitch, rubber, plastics are common examples of amorphous solids. It has many characteristics of crystalline solid such as shape, rigidity, and hardness.
They do not arrange orderly and melt gradually over a range of the temperature. Therefore, amorphous solids like Glass, pitch, rubber, plastics are called supercooled liquid rather than solid.
Difference between crystalline and amorphous solids
The main differences between crystalline and amorphous solids are,
- Crystalline solids have a definite structure with a sharp melting point but amorphous solids do not contain a definite structure with a sharp melting point.
- The constituents of crystalline solids are orderly arranged over a long-range in the crystal lattice. But in an amorphous solid, the constituents do not have an order arrangement.
Types of crystalline solids
Based on the nature of force operating between constituents (atoms, ions, molecules), the crystalline solids or crystals are classified into four types,
- Molecular crystalline solid
- Ionic crystalline solid
- Covalent crystals
- Metallic crystalline solid
Molecular crystalline solid
Forces that hold the constituents of molecular crystalline solid are weak Van der Waals type. The forces are two types like interatomic or intermolecular and originating from the dipole-dipole attraction.
Due to the presence of weaker forces, the molecular crystals are a soft and comparatively low melting point. The common examples of molecular crystalline solids are carbon dioxide, nitrogen, mica, borax, boric acid, most organic hydrocarbon like alkanes or alkenes.
According to polarity, the molecular crystalline solids are mainly three types,
- Non-polar molecules
- Polar molecules
- Hydrogen bonding molecules
Some molecular crystalline solid have a non-directional structure due to the absence of the dipole moment.
Hydrogen, helium, argon, oxygen, chlorine, carbon dioxide, methane molecule are examples of non-polar crystalline molecules. The forces operating between constituents are a weak London dispersion force.
A polar molecule is one in which one end of the molecule contains a positive charge and the other end contains a negative charge. Sulfur dioxide and ammonia are common examples of polar crystalline molecules. Due to electric polarization the constituent elements also binding by low forces of attraction.
Hydrogen bonding molecules
Hydrogen bonding is the weak type of chemical bonding due to very unstable attractive forces between a hydrogen atom and electronegative atoms like oxygen, nitrogen, and fluorine. A common example is seen in the dimer of formic acid or acetic acid.
The hydrogen bond is electrostatic but very weak with a bond energy of 5 to 6 kcal. The most discussed example of a hydrogen bonding crystalline solid is ice.
In ice, the 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 bonding crystalline solids.
Ionic crystalline solid
Forces involved in ionic crystalline solid are electrostatic in nature. These are strong and non-directional types. Therefore, ionic crystalline solid is strong and likely to be brittle.
They have very little elasticity and cannot be bent easily. The melting point in ionic crystals is high which is decreases with increasing the size of the ions.
Examples of ionic crystals
Sodium chloride (NaCl), potassium chloride (KCl), and magnesium chloride (MgCl2) are the most common examples of ionic crystals.
Calcium carbonate (CaCO3) is an example of an ionic crystal where some atoms are held together by covalent bonding.
In many crystals, the atoms in the structural units are held together by covalent bonding by pairing electrons of hybridized orbitals to form giant type molecules.
Examples of covalent crystals
Diamond, germanium, zinc sulfide, silver iodide, silicon carbide are the known examples of covalent crystals.
In a diamond, every carbon atom is covalently linking with the other four carbon atoms along the tetrahedron.
Metallic crystalline solid
Electrons are held loosely bound in these types of crystalline solid. They are good conductors of electric energy and thermal energy. Metallic crystalline solid is strong but can be bent.
The metal atom mainly formed cubic body-centered, face-centered, and hexagonal closed packed crystalline solid.
In hexagonal crystalline form, every metal atom is surrounded by 12 other metal atoms by the metallic bond with coordination number twelve. The forces are non-directional in nature. All the metals of periodic table elements are formed different types of crystals structure.
Crystalline allotropes of carbon
Allotropes of carbon have formed both types of solids, crystalline and amorphous in nature.
Diamond, Lonsdaleite, and graphite are examples of crystalline allotropes of carbon. The other rare and poorly understood allotropes of carbon are β-graphite, hexagonal diamond, Chaoite (very rare mineral), and carbon VI.
Structure of diamond
In diamond, each sp3 hybridized orbital of carbon is tetrahedrally surrounded by four other carbon atoms. The C-C bond distance in the diamond crystal is 154 pm. These tetrahedral structures form a cubic crystal unit of the diamond.
Structure of graphite
Graphite consists of the layer structure in each layer of the carbon atoms is sp2 hybridized. It is arranged in a hexagonal planner arrangement with free π-electrons.
The π-electrons are responsible for the electrical conductance in graphite. Successive layers of carbon atoms attached by weak van der Waals forces with separation of layers 335 pm.
Amorphous allotropes of carbon
Coal, carbon black, soot, carbide-derived carbon, and other impure forms rather than graphite and diamond are examples of amorphous solids.
All of these forms contain poly-crystalline structures. Therefore, amorphous carbon is the allotropes of carbon that do not form a crystalline solid structure.