Table of Contents
The constituent particles of solid material are kept together by strong forces of attraction, and as a result, the particles of solids are packed in appropriate order or symmetry. As a result, a solid has a clear form, a set volume, and low compressibility. Some solids are quite strong, whereas others may be broken by force.
A solid interface is described as a few atomic layers separating two solids in close contact, the characteristics of which differ considerably from those of the bulk substance it separates.
Solids are categorized into the following groups based on their crystal structures:
- Crystalline solids
- Amorphous solids
Crystalline solids: Crystals or crystalline solids are solids with sharp edges and well-defined planes. True solids are another name for crystalline solids. Crystalline solids include sodium chloride, quartz, gold, copper, and iron.
Properties of Crystalline solids:
- Long-range order exists in crystals.
- Crystalline solids have a sharp and distinct melting point.
- When a crystal is sliced with a sharp-edged instrument, it splits into two parts, and the freshly formed surfaces are flat and smooth.
- Crystalline solids are Anisotropic. Along various planes of the same crystal, they exhibit varied electrical and optical characteristics.
Amorphous solids: Amorphous solids are solids that lack a well-defined structure and form. Supercooled liquids and phantom solids are other names for amorphous solids. Amorphous solids include glass, rubber, and plastics.
Properties of Amorphous solids:
- Short-range order exists in amorphous substances.
- When heated, amorphous solids either break down or gradually transform into liquids across a wide temperature range.
- The cleavage of amorphous substances results in uneven cracks.
- Amorphous materials are isotropic, meaning they have the same electrical and optical characteristics in all directions.
Classification of Crystalline Solids
On the basis of the binding forces between their constituent particles, crystalline solids are divided into four major groups.
- Ionic Solids
The component particles of an ionic solid are cations and anions. These ions are grouped in three dimensions in a regular manner. The binding force in an ionic solid is the strong electrostatic force of attraction between cations and anions. In the crystal, the ions are organized alternately. In three-dimensional space, this sort of organization is repeated.
Ionic solids include sodium chloride (NaCl), potassium chloride (KCl), and lead bromide (PbBr2).
Characteristics:
- Ionic solids have the following features due to the strong electrostatic force of attraction:
- Ionic solids are rigid and brittle, with high melting temperatures and enthalpy of fusion.
- Ionic solids are solid-state insulators. However, they conduct electricity when molten and in aqueous solutions.
Metallic Solids
At room temperature, metals such as iron, copper, gold, silver, sodium, and potassium exist as solids. The atoms in a metallic crystal are bound together by a strong force of attraction known as metallic bonding. Metal occupies permanent locations in metallic crystals, although their valence electrons move.
Characteristics:
- They’re tough but malleable and ductile.
- They are excellent conductors of both heat and electricity.
- They are gleaming.
- Their melting and boiling points are both very high.
Covalent Solids
Covalent solids are solids in which covalent connections connect the atoms throughout the crystal. Covalent solids are also referred to as Atomic solids or Network solids or Macromolecular crystals or Gaint molecules.
Macromolecular crystals include asbestos, silicon carbide, mica, graphite, and diamond.
Characteristics:
- They are quite hard. Diamond, for example, is the hardest naturally occurring material (except graphite is soft).
- They have extremely high melting points.
- They are poor conductors of both heat and electricity (except graphite which is an atomic solid and is a good conductor of electricity).
- They have a high fusion heat.
Molecular Solids
These are solid substances in which the component particles are molecules. Molecular solids include iodine (I2), ice (H2O), dry ice (solid carbon dioxide), and naphthalene.
Nonpolar molecular solids, polar molecular solids, and hydrogen-bonded molecular solids are the three subgroups of molecular solids:
Non-Polar Molecular Solids
Nonpolar molecules are component particles in these sorts of materials. Van der Waals’ force of attraction, also known as the London dispersion force, is responsible for binding these molecules together. This molecular attraction force is weak.
Dry ice (solid (CO2)), iodine (I2), solid hydrogen (H2), naphthalene, and wax are among the examples.
Properties:
Nonpolar molecules have the following features due to the weak intermolecular force of attraction:
- They are soft.
- They also have a low melting point.
- They have a low fusion enthalpy.
- They are bad conductors.
- They are very flammable and have electrical insulators.
- Some nonpolar compounds solidify and emit a distinctive odour.
Polar Molecular Solids
Polar molecules with permanent dipole moments are component particles in such materials. The force that holds polar molecules together is known as dipole-dipole interaction, and it is significantly stronger than dispersion forces.
Solid ammonia and solid sulfur dioxide are two examples.
Properties:
- These solids are flexible.
- They are poor heat and electrical conductors.
- Their melting points are greater than those of nonpolar compounds.
- Their melting values are lower than those of ionic solids.
- Some molecular solids sublime and emit a distinct odour.
Hydrogen-Bonded Molecular Solids
This sort of bonding is feasible in any solids that have hydrogen atoms bound to extremely electronegative atoms such as F, O, or N. Because the crystals’ components are molecules, these solids are known as molecular solids.
Many organic molecules containing hydroxyls (OH) groups, such as phenols, alcohols, and carboxylic acids, crystallize via hydrogen bonding.
Properties:
- They are solids with a high melting point.
- They are poor heat and electrical conductors.
- They have a low melting point.
Classification of Solids Based on Band Theory (Electrical Conductivity)
The solids have been classed as follows based on their capacity to conduct current: i. Conductors; ii. Semiconductors; and iii. Insulators.
Metals have high conductivity, whereas insulators have a very low conductivity. Semiconductors are located between these two.
- Electric Current Conduction in Metals
The number of valence orbitals in a metallic atom is more than the number of valence electrons. As a result, electrons have greater room to migrate and conduct electricity.
According to this idea, metallic atoms’ atomic orbitals combine to generate molecular orbitals. A band is a collection of molecular orbitals, and the difference in energy between these orbitals is referred to as a gap. In metals, electrons partially occupy valence bands while leaving the conducting band of greater energy empty. In metals, the gap between the conduction band and the valence band is not particularly great. That is, if this band is only partially filled or overlaps with a higher energy unoccupied conduction band, electrons can readily move under the influence of the electric field and so conduct electricity.
- Electric Current Conduction in Semiconductors
The energy gap between the valence band and the conduction band in semiconductors is not particularly wide. As a result, valence electrons tend to leap into the conduction band. As a result, when an electric field is applied, some electrons migrate to the conduction band and begin to conduct electricity.
- Electric Current Conduction in Insulators
When the energy difference between the valence and conduction bands is particularly high, valence electrons are unable to leap into the conduction band. Be a result, they do not conduct electricity and are referred to as insulators.
Classification of Solids With Respect to Magnetic Properties
Solids are classed as diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, or ferrimagnetic based on their magnetic characteristics.
- Diamagnetism
This feature is generated by the orbital electron’s induced magnetic field. It is present in all substances to some extent, although it is more evident in materials such as H2, KCl, NaCl, and TiO2, where all electrons have paired spins. The diamagnetic compounds are weakly repellent to the magnetic field.
- Paramagnetism
It is caused by electron spin and orbital angular momentum. All solids with unpaired electrons, such as atoms, ions, or molecules, exhibit paramagnetism. Paramagnetic materials are weakly attracted to a magnetic field, and this is only visible when the magnetic field is present. Paramagnetism is seen in O2, Cr3+, Cu2+, and Fe3+.
- Ferromagnetism
Particles aligned on the lattice and electrons with parallel spins generate this feature. Ferromagnetic elements include iron (Fe), cobalt (Co), and nickel (Ni). A magnetic field attracts them quite strongly. At a specific temperature, it is a permanent characteristic of the material.
- Antiferromagnetism
Particles organized on two lattices with spins on one lattice antiparallel to those on the other lattice create this feature. As a consequence, magnetic moments on the lattice cancel out. Antiferromagnetic materials include MnO, MnSe, and KMnFe3.
- Ferrimagnetism
This magnetic feature is caused by particles on interpenetrating lattices with an uneven number of electrons and antiparallel spins. This is an example of a scenario having a net magnetic moment.
FAQ’s
Why do solids have a fixed volume?
The component particles of a solid are locked in their places and cannot move freely. They are abrasive. As a result, they have a fixed volume.
Ionic solids conduct electricity while molten, but how do they do so when solid?
Ionic solids dissolve to provide free ions in the molten state and hence conduct electricity; but, in the solid state, the ions are not free but are locked together by a strong electrostatic force of attraction and thus cannot conduct electricity.
