Question

Crystalline sodium chloride is a bad conductor of electricity because: (a) It contains free molecules (b) The ions present in it are not free to move (c) It does not possess ions (d) It contains only molecules

Short answer

(b) The ions present in it are not free to move.

Step-by-step solution

Read and Understand the Question

We are asked why crystalline sodium chloride is a bad conductor of electricity. Let's examine the choices to find the correct reason.

Evaluate Option (a)

Option (a) states that crystalline sodium chloride contains free molecules. However, sodium chloride forms a crystal lattice structure and does not contain discrete molecules. Hence, option (a) is incorrect.

Evaluate Option (b)

Option (b) says that the ions in crystalline sodium chloride are not free to move. In a solid state, the ions are fixed in a lattice and cannot move freely, making the solid a poor conductor of electricity. This option is likely correct.

Evaluate Option (c)

Option (c) claims that crystalline sodium chloride does not possess ions. This is incorrect because sodium chloride is an ionic compound, consisting of sodium and chloride ions.

Evaluate Option (d)

Option (d) suggests that sodium chloride contains only molecules. We know that it contains ions, not molecules, so this option is incorrect.

Select the Correct Option

After evaluating all options, we determine that option (b) is the correct answer. In crystalline sodium chloride, ions are not free to move, making it a poor conductor of electricity.

Key concepts

Crystalline Structure

Crystalline structures are an ordered arrangement of particles within a solid. These particles can be atoms, ions, or molecules. In the case of crystalline solids like sodium chloride (NaCl), the structure is composed of repeating units that form a lattice. This arrangement is highly organized and results in a rigid structure.
In crystalline sodium chloride, each sodium ion (\(\text{Na}^+\)) is surrounded by six chloride ions (\(\text{Cl}^-\)) and vice versa, creating a cubic lattice.

• The regular pattern ensures strong bonds between the particles, providing stability.
• This fixed structure allows these particles to be locked in place.

The strong electrostatic forces between the ions make the crystalline structure hard and brittle, keeping the ions stationary at room temperature. Therefore, while it forms a strong, stable arrangement, the inability of the ions to move freely renders crystalline sodium chloride non-conductive in its solid state.

Ionic Compounds

Ionic compounds are formed through the transfer of electrons between a metal and a non-metal, resulting in the formation of ions. These compounds consist of positive ions (cations) and negative ions (anions).
For example, sodium chloride is composed of sodium ions (\(\text{Na}^+\)) and chloride ions (\(\text{Cl}^-\)).

• Sodium (a metal) loses an electron to form \(\text{Na}^+\).
• Chloride (a non-metal) gains that electron, forming \(\text{Cl}^-\).

The attraction between oppositely charged ions creates a strong ionic bond, resulting in the formation of a solid ionic compound.
Ionic compounds typically form crystalline lattice structures at room temperature due to these strong ionic bonds. But, it is this very structure that inhibits the movement of ions, impacting properties such as electrical conductivity in the solid state.

Electrical Conductivity

Electrical conductivity in solids depends on the ability of charge-carrying particles to move freely. For a substance to conduct electricity, charged particles like ions or electrons must have the freedom to move under the influence of an electric field.
In the case of crystalline sodium chloride, the rigid structure of the crystal lattice restricts the movement of ions in its solid form.

• The fixed position of ions means they cannot participate in electrical conduction.
• While in solid state, there are no free-moving charge carriers.

However, when sodium chloride is dissolved in water or melted, the lattice breaks down, freeing the ions.
In liquid or dissolved state, NaCl becomes conductive because the ions can move, allowing them to carry electrical current. This demonstrates how changes in state can significantly affect the electrical properties of ionic solids.