The Solid State
Introduction
We know that liquids and gases are called fluids because of their ability to flow. The fluidity in both of these states is because the molecules are free to move about. On the contrary, the constituent particles in solids have fixed positions and
can only oscillate about their mean positions. This explains the rigidity of solids. These properties depend
upon the nature of constituent particles and the binding forces operating between them.
Different possible arrangements of particles resulting in several types of structures. Different arrangements of structural units lend different properties to solids.
Properties = f(structural arrangements)
Matter can exist in three states namely, solid, liquid, and gas. Under a given set of conditions of temperature and pressure, which of these would be the most stable state of a given substance depends upon the net effect of two opposing factors. States of matter = f(temperature, pressure). These are intermolecular forces that tend to keep the molecules (or atoms or ions) closer, and the thermal energy, which tends to keep them apart by making them move faster. At sufficiently low temperatures, the thermal energy is low and intermolecular forces bring them so close that they cling to one another and occupy fixed positions. These can still oscillate about their mean positions and the substance exists in a solid-state.
The following are the characteristic properties of the solid-state:
- They have definite mass, volume, and shape.
- Intermolecular distances are short.
- Intermolecular forces are strong
- Their constituent particles (atoms, molecules, or ions) have fixed positions and can only oscillate about their mean positions.
- They are incompressible and rigid.
Solids can be classified as crystalline or amorphous on the basis of the nature of order present in the arrangement of their constituent particles.
Crystalline solid
A crystalline solid usually consists of a large number of small crystals, each of them having a definite characteristic geometrical shape. The arrangement of constituent particles (atoms, molecules, or ions) in a crystal is ordered and repetitive in three dimensions. If we observe the pattern in one region of the crystal, we can predict accurately the position of particles in any other region of the crystal however far they may be from the place of observation. Thus, crystal has a long-range order which means that there is a regular pattern of arrangement of particles that repeats itself periodically over the entire crystal. Sodium chloride and quartz are typical examples of crystalline solids.
Amorphous solid
The term amorphous comes from the Greek word amorphos, meaning no form. The arrangement of constituent particles (atoms, molecules, or ions) in such a solid has only short-range order. In such an arrangement, a regular and periodically repeating pattern is observed over short distances only. Regular patterns are scattered and in between the arrangement is disordered. The structures of quartz (crystalline) and quartz glass (amorphous) are shown in Fig. (a) and (b).
In the case of amorphous quartz glass, there is no long-range order. The structure of amorphous solids is similar to that of liquids. Due to the differences in the arrangement of the constituent particles, the two types of solids differ in their properties.
Important aspects of crystalline solids and amorphous solids
👉 Crystalline solids have a sharp melting point. At a characteristic temperature, they melt abruptly and become liquid.
👉 Amorphous solids soften, melt and start flowing over a range of temperatures and can be moulded and blown into various shapes.
👉 Amorphous solids have the same structural features as liquids and are conveniently regarded as extremely viscous liquids. They may become crystalline at some temperature.
👉 Like liquids, amorphous solids have a tendency to flow, though very slowly. Therefore, sometimes these are called pseudo solids or super cooled liquids.
Isotropic Nature of Amorphous Solids
Amorphous solids are isotropic in nature. Their properties such as mechanical strength, refractive index and electrical conductivity, etc., are the same in all directions. It is because there is no long-range order in them and the arrangement of particles is not definite along all directions. Hence, the overall arrangement becomes equivalent in all directions. Therefore, the value of any physical property would be the same along any direction.
Anisotropic Nature of Crystalline Solids
Crystalline solids are anisotropic in nature, that is, some of their physical properties like electrical resistance or refractive index show different values when measured along different directions in the same crystals. This arises from different arrangement of particles in different directions. This is illustrated in Fig.
This figure shows a simple two-dimensional pattern of arrangement of two kinds of atoms. Mechanical properties such as resistance to shearing stress might be quite different in two directions indicated in the figure. Deformation in the CD direction displaces row which has two different types of atoms while in AB direction rows made of one type of atoms are displaced.
The distinction between Crystalline and Amorphous Solids
Classify crystalline solids on the basis of nature of intermolecular forces or bonds that hold the constituent particles together.
1. Molecular solids
2. Ionic Solids
3. Metallic Solids
4. Covalent solids.
1. Molecular solids
Molecules are the constituent particles of molecular solids. These are further subdivided into the following categories:
(i) Non-polar Molecular Solids: They comprise either atoms, for example, argon and helium or the molecules formed by non-polar covalent bonds, for example, `H_2, Cl_2` and `I_2`. In these solids, the atoms or molecules are held by weak dispersion forces or London forces. These solids are soft and non-conductors of electricity. They have low melting points and are usually in liquid or gaseous state at room temperature and pressure.
(ii) Polar Molecular Solids: The molecules of substances like `HCl, SO_2`, etc. are formed by polar covalent bonds. The molecules in such solids are held together by relatively stronger dipole-dipole interactions. These solids are soft and non-conductors of electricity. Their melting points are higher than those of non-polar molecular solids yet most of these are gases or liquids under room temperature and pressure. Solid `SO_2` and solid `NH_3` are some examples of such solids.
(iii) Hydrogen-Bonded Molecular Solids: The molecules of such solids contain polar covalent bonds between `H` and `F, O` or `N` atoms. Strong hydrogen bonding binds molecules of such solids like `H_2O` (ice). They are non-conductors of electricity. Generally, they are volatile liquids or soft solids under room temperature and pressure.
2. Ionic solids
Ions are the constituent particles of ionic solids. Such solids are formed by the three-dimensional arrangements of cations and anions bound by strong coulombic (electrostatic) forces. These solids are hard and brittle in nature. They have high melting and boiling points. Since the ions are not free to move about, they are electrical insulators in the solid-state. However, in the molten state or when dissolved in water, the ions become free to move about and conduct electricity.
3. Metallic Solids
Metals are orderly collection of positive ions surrounded by and held together by a sea of free electrons. These electrons are mobile and are evenly spread out throughout the crystal. Each metal atom contributes one or more electrons towards this sea of mobile electrons. These free and mobile electrons are responsible for the high electrical and thermal conductivity of metals. When an electric field is applied, these electrons flow through the network of positive ions. Similarly, when heat is supplied to one portion of the metal, the thermal energy is uniformly spread throughout by free electrons. Another important characteristic of metals is their lustre and colour in certain cases. This is also due to the presence of free electrons in them. Metals are highly malleable and ductile.
4. Covalent solids.
1.1 Why are solids rigid?
1.2 Why do solids have a definite volume?
1.3 Classify the following as amorphous or crystalline solids: Polyurethane, naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl chloride, fibre glass, copper.
1.4 Refractive index of a solid is observed to have the same value along all directions. Comment on the nature of this solid. Would it show cleavage property?
Work in progress...
References
- NCERT Chemistry Class XII
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Class 12