Short Answer

Molecular crystals are solids in which the particles present are molecules held together by weak intermolecular forces such as van der Waals forces, dipole–dipole forces, or hydrogen bonds. These forces are much weaker than ionic or metallic bonds, so molecular crystals are usually soft and have low melting points.

These crystals do not conduct electricity because they do not contain free ions or electrons. Common examples of molecular crystals include ice, dry ice (solid CO₂), iodine, sulphur, naphthalene, and sugar. Their physical properties mainly depend on the type and strength of intermolecular forces between the molecules.

Detailed Explanation

Molecular Crystals

Molecular crystals are a type of crystalline solid in which the repeating units found in the crystal lattice are molecules. These molecules are held together not by strong chemical bonds like ionic or covalent bonds, but by intermolecular forces that are relatively weak. These weak forces include London dispersion forces, dipole–dipole interactions, and hydrogen bonding. Since the molecules do not share electrons or form strong bonds with each other, molecular crystals have special physical properties that make them very different from ionic, covalent, or metallic crystals.

1. Composition and Structure of Molecular Crystals

In molecular crystals, the fundamental building blocks are neutral molecules.
These molecules can be:

• Non-polar molecules such as CO₂, I₂, CH₄
• Polar molecules such as H₂O, NH₃, HF

These molecules are arranged in a regular repeating pattern, forming a crystal lattice. Unlike ionic or metallic crystals, no electrons are transferred or delocalised. The molecules remain intact and unchanged.

The structure is stable because the molecules attract each other through weak forces. However, since these forces do not involve strong bonding, the molecules can separate easily when heated. This is why molecular crystals have low melting and boiling points.

2. Types of Intermolecular Forces in Molecular Crystals

The stability and properties of molecular crystals depend on the type of intermolecular force between the molecules.

London Dispersion Forces

These temporary forces occur in:

• Non-polar molecules
• Noble gases

They are the weakest type of force.

Examples: Solid argon, naphthalene, iodine crystals.

Dipole–Dipole Forces

These occur between:

• Polar molecules
• Molecules that have permanent dipoles

Dipole–dipole interactions are stronger than dispersion forces.

Examples: Solid hydrogen chloride, solid ammonia.

Hydrogen Bonding

This is the strongest intermolecular force found in molecular crystals.
Hydrogen bonding occurs when hydrogen is bonded to:

• Oxygen
• Nitrogen
• Fluorine

Examples: Ice (solid water), solid HF.

These different forces cause differences in melting point, hardness, and solubility among molecular crystals.

3. Physical Properties of Molecular Crystals

Low Melting and Boiling Points

Molecular crystals melt easily because weak forces hold the molecules together. Only a small amount of heat can separate them.
Example: Naphthalene melts easily when heated.

Softness

These crystals are soft and easily broken because the intermolecular forces are weak.
Example: Iodine crystals break or powder easily.

Poor Conductors of Electricity

Molecular crystals do not conduct electricity because:

• They have no free electrons
• They do not contain ions

Example: Ice does not conduct electricity in solid state.

Generally Volatile

Many molecular crystals easily evaporate or sublime.
Example: Dry ice (solid CO₂) directly turns into gas.

Often Low Density

Since molecules are loosely packed, they have lower density compared to ionic or metallic crystals.

4. Examples of Molecular Crystals

Common examples include:

• Ice (H₂O) – held by hydrogen bonds
• Dry ice (CO₂) – held by dispersion forces
• Iodine (I₂) – held by dispersion forces
• Naphthalene – an aromatic compound in mothballs
• Sulphur – made of S₈ molecules
• Sugar (glucose, sucrose) – held by hydrogen bonds

Each of these has unique physical properties depending on the strength and type of intermolecular forces.

5. Comparison With Other Crystals

Molecular crystals differ from other types of crystals:

• Ionic crystals have very high melting points because strong electrostatic forces hold ions together.
• Covalent crystals like diamond have networks of covalent bonds, making them extremely strong.
• Metallic crystals have delocalised electrons, giving them conductivity and strength.

Molecular crystals are the weakest among all crystal types due to their weak intermolecular forces.

6. Applications and Occurrence

Molecular crystals are found in many natural and industrial materials:

• Ice forms naturally in the environment.
• Dry ice is used for cooling and preserving food.
• Naphthalene is used in mothballs and repellents.
• Organic compounds with low melting points often appear as molecular crystals.

Their softness, low melting points, and volatility make them useful in certain chemical processes.

Conclusion

Molecular crystals are solids made of molecules held together by weak intermolecular forces such as dispersion forces, dipole–dipole interactions, and hydrogen bonding. They are soft, melt easily, have low density, and do not conduct electricity. Examples include ice, dry ice, iodine, sulphur, and naphthalene. Their structure and properties depend on the type of intermolecular forces present, making them an important category of crystalline solids in chemistry.