Short Answer

The electrical conductivity of crystals depends on the presence and movement of charged particles such as electrons or ions. If a crystal has free electrons or mobile ions, it can conduct electricity. Crystals without these moving charges do not conduct electricity.

Metallic crystals conduct electricity because they contain free electrons, while ionic crystals conduct only in molten or solution form because ions become free to move. Covalent and molecular crystals generally do not conduct electricity because they do not have charge carriers.

Detailed Explanation

Electrical Conductivity of Crystals

Electrical conductivity refers to the ability of a crystal to allow electric current to pass through it. For electricity to flow, there must be charged particles that can move freely. These charged particles may be electrons or ions. Different types of crystals have different levels of electrical conductivity because they differ in bonding, structure, and availability of charge carriers.

Some crystals, like metals, conduct electricity very well, while others, like sugar or diamond, do not conduct electricity at all. The main factor that determines conductivity is whether the crystal has free-moving charged particles.

1. Role of Free Electrons

In many crystals, especially metallic crystals, conductivity is controlled by the presence of free or delocalised electrons.

Metallic Crystals

Metallic crystals are made up of metal atoms arranged in a regular lattice. Each metal atom releases one or more electrons, which move freely throughout the crystal.
These free electrons act as charge carriers.

Because electrons can move easily:

• Metals are good conductors
• Conductivity increases as electron mobility increases
• Examples: copper, aluminium, gold, silver

These metals are used in wires, cables, and circuits because they allow electric current to pass through them without much resistance.

2. Role of Mobile Ions

In some crystals, like ionic crystals, the ability to conduct electricity depends on the movement of ions.

Ionic Crystals

Ionic crystals are made of positive and negative ions held by strong electrostatic forces.

Examples:

• Sodium chloride (NaCl)
• Potassium chloride (KCl)
• Calcium oxide (CaO)

In solid state:

• Ions are fixed in place
• They cannot move
• Electricity cannot pass through the solid

In molten state or dissolved in water:

• The ions become free
• They can move and carry charge
• The substance becomes a good conductor

Therefore, ionic crystals conduct electricity only when:

• Molten (liquid state)
• In aqueous solution (ionic solution)

3. Covalent Crystals and Conductivity

Covalent crystals generally do not conduct electricity because:

• They do not have free electrons
• They do not have mobile ions
• All electrons are involved in covalent bonding

Examples:

• Diamond
• Silicon dioxide (quartz)

However, there are exceptions like graphite, which conducts electricity because:

• It has delocalised electrons between layers
• These electrons move easily and carry current

Graphite is used in electrodes and batteries because of this property.

4. Molecular Crystals and Insulating Nature

Molecular crystals are poor conductors because:

• They consist of neutral molecules
• They do not have free electrons
• They do not have ions
• Intermolecular forces are weak and cannot support charge flow

Examples:

• Ice
• Sugar
• Naphthalene
• Iodine

These crystals act as insulators.

5. Band Theory and Conductivity (Simple Explanation)

In solids, electron movement depends on the arrangement of energy levels:

• Conductors have overlapping valence and conduction bands.
  → Electrons move easily.
• Semiconductors have a small gap between bands.
  → Some electrons can jump if given energy.
  Example: silicon, germanium.
• Insulators have a large gap between bands.
  → Electrons cannot move.
  Example: diamond.

Band structure determines how easily electrons can move through the crystal.

6. Temperature and Conductivity

Temperature affects different crystals in different ways:

Metals

• Conductivity decreases with increase in temperature
• Heat causes vibrations that block electron movement

Ionic Crystals

• Conductivity increases when temperature helps ions separate
• High temperature melts the crystal, allowing ion mobility

Semiconductors

• Conductivity increases as temperature increases
• More electrons gain energy and move to conduction band

Temperature plays a major role in determining conductivity behaviour.

7. Presence of Impurities

Impurities can change conductivity:

• In semiconductors, adding impurities increases conductivity (doping)
  Example: Adding phosphorus to silicon
• In metals, impurities generally reduce conductivity
• In ionic crystals, impurities may reduce or increase conductivity depending on ion type

Conclusion

The electrical conductivity of crystals is determined mainly by the presence and movement of charged particles such as free electrons or mobile ions. Metallic crystals conduct electricity due to delocalised electrons, while ionic crystals conduct only in molten or solution form when ions become free. Covalent and molecular crystals are generally insulators because they lack mobile charge carriers. Temperature, band structure, and impurities also affect conductivity, making this property unique for each type of crystal.