Amorphous and Crystalline Solids for IIT JEE

Matter can be subdivided into three states – solid, liquid, and gas. Another classification of the matter is condensed state and gaseous state where the condensed state 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 characterised by two types of solids – amorphous solids and crystalline solids. These two types are discussed, in detail, below.

Amorphous Solids

Amorphous solids are structures that are closely packed 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. The most common example of an amorphous solid is glass. Also, other good examples of amorphous solids are gels, plastics, various polymers, wax, and thin films.

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. Due to such an arrangement of the molecules, amorphous solids differ from general solids. 

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

The reticular or granular structure in these types of solids is absent. This is because of the orderliness present over a short range compared to normal solids. Examples are glass and plastic. 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 acting like any ordinary viscous liquids. Any long range orderliness is absent in amorphous solids; hence, 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, and therefore used in rapidly degrading 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 very high viscosity (resistance to flow). Most solids tend to exist in the crystalline state rather than in the amorphous state because there is always 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 molecular structure is so complex that it hey cannot easily be rearranged to form a crystalline structure and/or

2. Formation of the solid happens 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-dimensional 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 orderliness and thus has definite properties such as a sharp melting point. So, we can say that a 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 a 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

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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 Supercooled Liquids is another name for Amorphous Solids.

A clear difference between amorphous and crystalline solids is clearly visible in the image given below:

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In this article, we have discussed about amorphous solids and crystalline solids. We have also learned about the differences between amorphous solids and crystalline solids. We hope that this article will be helpful to understand the underlying concepts while preparing for the IIT JEE exam.