The solid-state chemistry is one of the booming areas of science, especially because of exciting new developments in superconducting and semiconducting solids and other new materials like biodegradable polymers for packaging, bio compliant solids with widely different properties.
General Characteristics of Solid State
According to Kinetic theory, the physical state in which a substance exists is determined by the resultant of the disordering effect of thermal energy and ordering effect of the cohesive forces of attraction as follows:
- If the thermal energy is much greater than the forces of attraction, then we have matter in the gaseous state.
- If the forces of attraction are greater than the thermal energy, matter exists in liquid state.
- If the forces of attraction between molecules are much greater than the thermal energy, the matter exists in the solid state.
The cohesive forces which operate between ions in an ionic compound are strong electrostatic forces. Therefore, ionic compounds are generally solids.
The forces which operate between molecules in a covalent compound are weak intermolecular or Van der Waals’ forces.
The attractive forces operating in case of atoms or molecules of non-metallic elements are also Van der Waals’ forces. Then, such substances exist as gases, liquid, or low melting solids.
The following are the characteristic properties of the solid state:
- Intermolecular distances– They have short intermolecular distances.
- Intermolecular forces- Intermolecular forces in solids are very strong.
- Molecular Motion– Their constituent particles (atoms, ions, or molecules) have fixed positions and can only oscillate about their mean positions.
- Define Shape and Volume- Constituents units (atoms, ions, or molecules) in a solid are held together by strong attractive forces. Thus, constituent units in a solid occupy fixed positions due to lack of translatory motion and strong attractive forces, solids have fixed volume and shape.
- Rigidity– Solids are hard and rigid. This is because of the strong attractive forces between the constituent units due to which they have fixed positions.
- High Density- Solids have a higher density as compared to gases and liquids. This is because the constituent units in solids are closely packed leading to a decrease in volume. Hence, mass per unit volume or density is high.
- Compressibility– Solids are nearly incompressible. Due to the close packing of constituent units, free space between them gets eliminated to a large extent and hence they can’t be compressed.
- Diffusion– Diffusion of solids is almost negligible. This is because the constituent units in a solid have fixed positions from which they can’t move appreciably.
The temperature at which a solid melt under one atmospheric pressure is called the Melting point of the solid.
Crystalline and Amorphous solids
A crystalline solid is a substance whose constituent units are arranged in an orderly fashion which repeats itself over very long distances as compared to interatomic distances, i.e., they have a long-range order.
Some common examples of crystalline solids are sodium chloride, quartz, diamond, copper, potassium nitrate, urea, naphthalene and benzoic acid.
An amorphous solid is a substance whose constituent units do not possess an orderly arrangement over a long range. The orderly arrangement in them is restricted to very short distances which are the same order of magnitude as the interatomic distances, i.e., they have a short-range order.
Some common examples of amorphous solids are glass, plastics such as PVC, Teflon, cellophane, polyurethane, quartz glass, etc.
Sometimes a part of amorphous solids is crystalline and the rest is non-crystalline.
Difference between Quartz and Quartz Glass
Difference between Crystalline and Amorphous Solids
The constituent units of molecular solids are small molecules, except in solidified noble gases where the units are atoms.
These are of Two types-
- Non-polar Molecular Solids
- Polar Molecular solids
- Hydrogen Bonded Molecular Solids
The constituent units in ionic solids are oppositively charged ions. The constituents in an ionic solid are held together by strong electrostatic force of attraction. Examples of ionic solids are NaCl, CsCl, and BaCl2.
The constituent units of metallic solids are positive ions. The positively charged metal ions are held together by the free-moving electron charge which arises due to the grouping of all the valence electrons. The electron charge cloud belongs to the entire crystal.
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