A) Solid State- Introduction
A matter can exist in four states- solid, liquid, gaseous and plasma. The main property that differentiates the solid from the other states of matter is that it possesses rigidity or hardness. The liquids and gases are not hard in nature and have an ability to flow, therefore, possesses fluidity.
Solids have a definite mass, shape and volume whereas Liquids have a definite volume but no definite shape. It takes the shape of the container it occupies. Gases have no definite shape and no definite volume.
Solids being hard in nature, it cannot be compressed or pressed into smaller shapes. In short, the solids are incompressible. They can only be cut into smaller shapes. The constituent particles that form the solid structure do not move from its place and occupy fixed positions. It is the strong intermolecular forces that are responsible for holding these molecules in place. The distance between these particles that is their intermolecular distance is also less.
Although these molecules occupy fixed positions, but these molecules are not stationary. They show motion known as the vibrational motion. These particles oscillate or vibrate about their mean position like the pendulum going from side to side constantly. This vibrational energy that solids possess is less. But if this energy is increased by supplying the solid with thermal or the heat energy, in that case, the increase in temperature can increase the vibrations and weaken the intramolecular forces. The particles will then become free and go into the liquid and the gaseous state. Therefore, low thermal and vibrational energy preserves the solid state.
To summarize; the characteristic properties that define the solid state are:
1) Solids have definite shape, mass and volume
2) They are rigid and incompressible
3) The constituent particles (that is atoms, ions or molecules) occupy fixed positions
4) Strong intermolecular forces hold the constituent particles in one place
5) The intermolecular distance between the particles is less
6) The particles do not remain stationary but oscillate about their mean positions. In other words, the constituent particles have low vibrational energy and low thermal energy.
All these features are responsible for a substance to exist as a solid.
B) Types of Solids-
i) Introduction to Crystalline and Amorphous Solids
Solids are made up of a vast number of particles. Depending on how the particles are arranged, solids can be divided into two categories- crystalline solids and amorphous solids. The particles may be in the form of atoms, ions or molecules.
For example; gold atoms, NaCl ions, and sucrose molecules give solids- gold, salt, and sugar respectively.
ii) Formation and Arrangement- If a solid is heated to a particular high temperature and the molten liquid is gradually cooled, the constituent particles get sufficient time to arrange themselves into a highly ordered crystalline solid structure. This process of heating a solid to a particular high temperature followed by the gradual cooling is called annealing, and it can affect the crystalline property of a solid.
Examples of crystalline solids are Diamond, NaCl, and Quartz
If the molten liquid is not cooled gradually but is cooled rapidly or vapors of the liquid are frozen suddenly then, the particles do not get enough time to organize. The arrangement is then disordered forming an amorphous solid. Some of the examples of amorphous solids are Rubber, Glass, Tar, Plastic and Quartz glass.
In short, it is the cooling process of the molten liquid to solid that decides whether a crystalline solid is obtained or an amorphous.
It is important to note that an ideal, perfect crystalline solid can only be obtained at 0K because particles do not have any thermal energy at this low temperature. There is no disorder, the entropy of the entire system is zero, and the particles can arrange themselves in perfect order. At all other temperatures, small irregularities always exist in the crystal structure and we do not obtain a perfect crystalline solid.
The smallest building block of a crystal is called a unit cell. In a three-dimensional diagram, a unit cell is shown as a box with corners. The atoms, ions or molecules that make up the crystalline solid structure occupy these corners of the box. The atoms may also be present at the centre of the unit cell, on the faces of the unit cell or at the edges of the unit cell in addition to the corners. These places that atoms occupy in a unit cell are called the lattice points. Multiples of unit cells repeat to give a crystal lattice. It is called a lattice because, in the two dimensions, a crystal looks like a series of points with criss-cross lines like a network or mesh.
When the entire crystal is shown as lines and points in the three dimensions, it's called a crystal lattice. As the unit cells extend in all the three dimensions in space, therefore, a crystal lattice is also known as a space lattice. These repeating structures of the unit cell in all the three dimensions give an entire crystalline solid.
Let me summarize this concept for you in brief. The unit cell is the basic unit of a crystalline solid. When the unit cells repeat periodically in all the direction maintaining the order of the arrangement of the particles, it gives a crystalline solid. A crystal lattice shows a part of the crystalline solid as lines and points denoting the three-dimensional arrangement of the unit cells.
We can understand this better by comparing a crystalline solid with a living being. The biological cell is the basic unit of all the living organisms. Similarly, the unit cell is the basic unit of a crystalline solid. As the biological cells comes together to give multi-cellular organism. Similarly, many unit cells come together in all the three dimensions to give a crystalline solid.
iii) Order- Crystalline solids are therefore said to have both short range and long range order that is the arrangement of the constituent particles are ordered and regularly repeat over a smaller region and also over an extended area. Due to this repeating arrangement, we can predict the structure of the entire crystal if the structure of one of the unit cell is known.
Amorphous solids do not have long-range order but may have short range order. It means that there may be small parts in the solids which may have an ordered arrangement of the constituent particles. There may exist small scattered regions having tiny crystals similar to the crystalline solids. Such crystals present in an amorphous solid are called crystallites.
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