Crystalline Solids:

Amorphous solids are very useful but most of the solids are crystalline in nature due to thermodynamically favorable conditions. Crystalline solids are classified into basically four categories; based on the intermolecular forces operating between the constituents.

1. Ionic Solids
2. Metallic Solids
3. Covalent solids
4. Molecular Solids

1 - Ionic Solids – Solids, in which ions are the constituent particles, are called ionic solids. These solids are characterized by cationic and anionic species that are associated through electrostatic interactions.These solids are formed because of three dimensional arrangements of cations and anions bound together with strong electrostatic forces (coulombic forces). For example NaCl. Although oppositely charged ions have attractive interactions, like charges repel one another. In the determination of the lattice energy, U, the sizes and charges of the ions are most important.

Characteristics of Ionic Solids –

• High melting and boiling points.
• Non-conductor of electricity in solid state.
• Conductor of electricity in molten state.
• Conducted electricity when dissolved in water.

It is noteworthy that the calculated lattice energy is quite often smaller than

the empirical value. Whereas the ions in purely ionic compounds may be accurately

treated as hard spheres in the calculation, there is often a degree of covalency in the bonding molecule.

2 - Metallic Solids – All metals are referred as Metallic solids. Their constituent particles are positive ions. These positive ions are surrounded by free moving electrons. For example – iron, aluminium etc. Metallic solids are characterized by physical properties such as high thermal and electrical conductivities, malleability, and ductility (i.e., able to be drawn into a thin
wire). Chemically, metals tend to have low ionization energies that often result in
metals being easily oxidized by the surrounding environmental conditions.

Characteristics –

• High melting points.
• Good conductors of electricity and heat.
• Lustrous, and are of specific colors.
• Hard but malleable and ductile in nature.

The close chemical association among neighboring metal atoms in the solid
gives rise to physical properties such as high melting points and malleability. The
non-directional bonding in metals allows for two modes of deformation to occur
when a metal is bent. Either the atomic spacing between neighboring metal atoms in
the crystal lattice may change (elastic deformation), or planes of metal atoms may
slide past one another (plastic deformation). Whereas elastic deformation results in a
material with “positional memory” (e.g., springs), plastic deformation results in a
material that stays malformed.

3 - Covalent Solids – Crystalline solids are formed by non metals because of formation of covalent bonds between the adjacent molecules throughout the crystal. These are also known as Network Solids. These are also called giant molecules. These solids are characterized by very strong, directional covalent bonds between their constituent atoms. For example – diamond, graphite, silicon carbide, etc.

Characteristic of Covalent Solids –

• They are very hard and brittle except graphite which is soft.
• Very high melting points.
• Do not conduct electricity except graphite.
• Also called giant molecules

Due to the arrangement of the atoms comprising these solids,
a variety of physical properties may be observed, as shown by the very different
properties exhibited by the three discrete structural forms of carbon. For instance, diamond is an extremely hard, insulating material that is transparent to light, whereas graphite is a soft, black solid that is capable of conducting electricity along the graphitic layers of the extended solid. Buckminsterfullerene (C60) is very different from either of these carbon forms, being soluble in aromatic solvents, and thereby capable of undergoing chemical reactions.

4 - Molecular Solids – Solids having molecules as their constituent particles are called Molecular solids. For example Hydrogen, Chlorine, Water, HCl, solid carbon dioxide, sucrose, etc.

This class of solids features discrete molecules that are held together by rather weak
intermolecular forces such as dipole–dipole, London Dispersion, and hydrogen
bonding. Since these forces are much weaker than ionic or metallic bonding
interactions, molecular solids are usually characterized by low melting points. Examples include dry ice (CO2), ice (H2O), solid methane (CH4), sugar (comprising various arrangements/conformations of C6H12O6 molecules), and polymers.

Molecular solids are classified into three types on the basis of their bond:

1. Non-Polar Molecular solids
2. Polar Molecular Solids
3. Hydrogen Bonded Molecular Solids

1. Non Polar Molecular Solids – Solids which are comprised of atoms only, such as helium and argon or molecules; formed only because of the non polar covalent bonds are known as Non-Polar Molecular Solids. For example – H₂, Cl₂, I₂, etc.

 

Characteristic of Non-Polar Molecular Solids –

• The molecules of non-polar molecular solids are held together by weak dispersion forces or London forces.
• Non-Polar Molecular Solids are soft.
• Non-polar molecular solids are non-conductor of electricity.
• Non-polar molecular solids have low melting points.
• Non-polar molecular solids are usually in liquid or gaseous state at the room temperature and pressure.

(b) Polar Molecular Solids – The solids which are formed by polar covalent bonds are known as Polar Molecular solids. These molecules are held by dipole-dipole interactions. For example – HCl, SO2, NH3, etc.

Characteristic of Polar Molecular Solids –

• The molecules in polar molecular solids are held together with dipole-dipole interactions.
• Polar molecular solids are generally soft in nature.
• Polar molecular solids are non-conductor of electricity.
• Polar molecular solids have higher melting points in comparison to non-polar molecular solids.
• Most of the polar molecular solids are gases or liquids at room temperature and pressure.
• Solid SO2 and solid NH3 are some examples of polar molecular solids.

(c) Hydrogen bonded Molecular Solids – The molecules of hydrogen bonded molecular solids contain polar covalent bond between H and O, F or N. In solids such as H2O (ice) molecules are bound together strongly with hydrogen bond. HF, H2O (ice), etc are the examples of hydrogen bound molecular solids.

Characteristics –

• Hydrogen bound molecular solids are generally volatile liquid or soft solids at room temperature and pressure.
• Hydrogen bound molecular solids are non-conductor of electricity.

Crystal  Classification	Unit Particles	Binding Forces	Properties	Examples
Atomic	Atoms	London dispersion forces	Soft, very low melting, poor thermal and electrical conductors	Noble gases
Molecular	Polar or  non – polar molecules	Vander Waal’s forces (London dispersion, dipole – dipole forces hydrogen bonds)	Fairly soft, low to moderately high melting points, poor thermal and electrical conductors	Dry ice (solid, methane
Ionic	Positive and negative ions	Ionic bonds	Hard and brittle, high melting points, high heats of fusion, poor thermal and electrical conductors	NaCl, ZnS
Covalent	Atoms that are connected in covalent bond network	Covalent bonds	Very hard, very high melting points, poor thermal and electrical conductors	Diamond, quartz, silicon
Metallic Solids	Cations in electron cloud	Metallic bonds	Soft to very hard, low to very high melting points, excellent thermal and electrical conductors, malleable and ductile	All metallic elements, for example, Cu, Fe, Zn