Amorphous and Crystalline Solids for IIT JEE

Differentiation between Crystalline and Amorphous Solids

Matter can be subdivided into three states- solid, liquid and gas. Another classification of matter is – condensed stated and gaseous state where the former is further sub-classified into the solid state and the liquid state. Although very little of the matter is in the solid state in the universe, solids constitute much of the physical world around us and a large part of modern technology is based on the special characteristics of the different solid materials.

Solid state is generally characterized by two types of solids – amorphous solids and crystalline solids. These two types are discussed, in detail, as follows:

Amorphous Solids

Amorphous solids are structures that are rigid but lack a well - defined form. They have no geometric shape. Hence, they are not crystalline. This is why, like crystals, they don't have edges. An amorphous solid's most common example is Glass. Also other good examples of amorphous solids are gels, plastics, various polymers, wax and thin films.

Due to the arrangement of their molecules, this variation in solid characteristics occurs. Here the particles of matter do not form the three-dimensional lattice structure that we see in solids. Some amorphous solids that occur naturally have impurities that prevent the formation of such a structure. So they've got a molecular arrangement of short order.

Amorphous solids break up with irregular edges into uneven pieces. And they have no distinct molecular arrangement or shape. So, their structure cannot identify with those of crystals.

There is no reticular or granular structure in these types of solids. The causes exhibit short range orderness in their structure. Examples of this class are glass and plastic. We are talking about amorphous substances when the size of the grains or crystallites becomes comparable to the size of the pattern unit. A typical characteristic of these substances is that they do not have any specific melting points. They gradually become soft as their temperature increases; their viscosity decreases and they begin to act like ordinary viscous liquids. There is no long - range order of amorphous solids. There is no periodic location of the atoms or molecules in these solids over large distances.

Amorphous solids become important due to their applications in the following situations: 

  • • They are more soluble than crystalline forms. This property is useful for delivering poorly soluble drugs in pharmaceuticals.

  • • They are often chemically less stable, therefore used in rapid degradation chemical reactions.

  • • Sometimes they are the only form in which certain solids occur.

  • Many amorphous materials have liquid - like internal structures. In fact, the only obvious distinction between amorphous materials, such as glass, and liquids, is that the amorphous solids possess high viscosity (resistance to flow). All solids tend to exist in the crystalline state rather than in the amorphous state because there is always a greater binding energy in the crystalline structure. However, when liquids are cooled below the melting temperature, amorphous solids are formed in numerous instances. There are two reasons which explain this:

  • 1. The molecules' structure is so complex that they cannot easily be rearranged to form a crystalline structure and/or

  • 2. Solid forms so quickly that atoms or molecules do not have enough time to rearrange themselves in a crystalline structure.

  • Generally, amorphous solids have one of two distinct atomic arrangements: either a tangled mass of long-chained molecules or a 3-dimentional network of atoms with no long-range order.

    Solids not having an atomic order of long range are called amorphous solids. They often have subunits with consistent shape, but their long - range order is disturbed by the random packaging of the subunits. Amorphous solids are formed when liquids are cooled from the molten state too quickly to allow the sub - units to be arranged in a crystalline low - energy state. No amorphous solids are formed by solids with pure ionic bonds, but all other types of bonds can produce amorphous solids. Silica (SiO2) can form either covalent amorphous solids (usually called glasses) or regular crystal structures (Quartz).

    Crystalline Solids

    Generally speaking, a solid is said to be a crystal if the constituent particles (atoms, ions or molecules) are arranged periodically in three dimensional ways or simply have a reticular structure. The atoms are stacked regularly in crystalline solids, forming a 3-D pattern that can be obtained through a 3-D repetition of a particular pattern unit. It has long-range orderness and thus has definite properties such as a sharp melting point. So we can say that crystal is a periodic array of atoms in three dimensions. The external geometric shape of the crystal often remains unchanged when the crystal grows under a constant environment. The shape is therefore a consequence of the internal arrangement of the component particles. An infinite 3D repetition of identical units, which can be atoms or molecules, is the ideal crystal. All ionic solids are crystalline, as are most covalent solids. Under normal circumstances, all solid metals are crystalline.

    Types of crystalline solids:

  • • Ionic Crystals

  • • Metallic Crystals

  • • Molecular Crystals

  • • Covalent or Network Crystals

  • • Group VIII Crystals (frozen Noble Gases)

  • Crystal Structure

    When a group of atoms or molecules is attached identically to each lattice point, a crystal structure is formed. This group is called the basis of atoms or molecules. The crystalline lattice can be reproduced in three dimensions by translating the unit cell. The unit cell is the unique part of the crystal structure that generates the entire crystal structure when translated along parallel lines.

    The crystal structure system contains seven different types of crystals differentiated on the basis of their axes (a, b and c) and angles (α, β and γ):

  • • Cubic, with a = b = c and α = β = γ = 90o

  • • Tetragonal, with a = b ≠ c and α = β = γ = 90o

  • • Orthorhombic, with a ≠ b ≠ c and α = β = γ = 90o

  • • Monoclinic, with a ≠ b ≠ c and α = β ≠ γ = 90o

  • • Triclinic, with a ≠ b ≠ c and α ≠ β ≠ γ ≠ 90o

  • • Rhombohedral, with a = b = c and α = β = γ ≠ 90o

  • • Hexagonal, with a = b ≠ c and α = β = 90o, γ = 120o

  • Lattice, Lattice parameter and Lattice constant

    A lattice is a regular periodic array of space points that replace objects with imaginary points. It may be considered to remove the atom, but there is still the center. The Bravais and the non - Bravais are two classes of lattices. All lattice points are equivalent in a Bravais lattice and therefore necessarily all atoms in the crystal are of the same type. On the other hand, some of the lattice points are not equivalent in a non - Bravais lattice. Non - Bravais lattices are often referred to as base-based lattices. The basis is a set of atoms which is located near each site of a Bravais lattice.

    The constant lattice (or lattice parameter) refers to the constant distance in a crystal lattice between unit cells. Three - dimensional lattices usually have three constants of lattices, referred to as a, b, and c. However, all the constants are equal in the special case of cubic crystal structures and we only refer to one. Similarly, the constants a and b are equal in hexagonal crystal structures, and we only refer to the constants a and c. A group of constants of the lattice could be called parameters of the lattice. However, the full set of parameters of the lattice consists of the three constants of the lattice and the three angles.
    Bravais lattice is an infinite array of points with an arrangement and orientation that looks exactly the same from any lattice point. Following are the 14 Bravais lattices that exist:

    Out of these 14 Bravais lattices, the three cubic lattices are very commonly studied viz. the simple cubic, body centered cubic and the face centered cubic. The characteristic properties of each of these cubic lattices are listed in the following table:
    Unit CellAtoms per cellPacking FractionLattice ConstantCoordination Number
    Simple Cubic1 52%2r6
    Body-Centered Cubic2 68%4r/31/28
    Face-Centered Cubic4 74%2r21/212

    Differentiation between Crystalline and Amorphous Solids

  • • Crystals have their constituent particles orderly arranged. In comparison, there is no such arrangement for amorphous solids. Their particles are randomly-arranged.

  • • With definite edges, crystals have a specific geometric shape. Amorphous solids do not follow any definite geometry.

  • • Crystalline solids have a sharp melting point where they are definitely going to melt. An amorphous solid will melt over a temperature range, but no definite temperature.

  • • Crystals have their particles arranged in a long order. This means that the particles will indefinitely display the same arrangement. Amorphous solids are arranged in a short order. Their particles in the arrangement show variety.

  • • In particular points and directions, crystalline solids cleave (break). Amorphous solids with ragged edges divide into uneven parts.

  • • Crystals are known as True Solids, while Super-Cooled Liquids is another name for Amorphous Solids.