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

The correct answer is (b): the ions present in it are not free to move.

Step-by-step solution

Understand the Structure of Sodium Chloride

Sodium chloride, also known as table salt, is made up of sodium ions, \( \text{Na}^+ \), and chloride ions, \( \text{Cl}^- \), arranged in a crystalline lattice structure. This arrangement is stable and forms a solid.

Identify the Nature of Ion Movement in Solids

In a solid crystalline lattice like that of sodium chloride, the ions are fixed in their positions within the lattice structure. They vibrate slightly around their positions but do not have the freedom to move throughout the solid as they would in a liquid or gas.

Determine the Factors Required for Conductivity

Electrical conductivity requires the movement of charged particles. For solid materials, this generally means that the charged particles (ions or electrons) must be free to move. In sodium chloride, the ions are not free to move because they are held within the rigid lattice structure.

Analyze the Options Given

Given the options: (a) free molecules, (b) ions that are not free to move, (c) absence of ions, (d) only molecules present, we need to assess which reflects the actual scenario. We know that sodium chloride is ionic and contains ions, which are not free to move in its solid state.

Choose the Correct Answer

By understanding that sodium chloride in its solid state contains ions that are not free to move, we identify that option (b) best describes the reason why crystalline sodium chloride is a bad conductor of electricity.

Key concepts

Sodium Chloride Structure

Sodium chloride, commonly known as table salt, has a distinctive crystal structure. This structure is known as a face-centered cubic (FCC) lattice. Each sodium ion, denoted as \( \text{Na}^+ \), is surrounded by six chloride ions, \( \text{Cl}^- \), and vice versa. This orderly lattice extends throughout the solid, creating a highly stable configuration.

The stability of this structure is due to the strong electrostatic forces of attraction between the oppositely charged ions. These forces hold the ions in fixed positions, resulting in a rigid and firm crystalline form. This arrangement is not only stable but also requires significant energy to break apart, contributing to the compound's high melting and boiling points.

• The cube shape is easy to observe in natural salt crystals.
• This structural arrangement is key to understanding both the physical properties and lack of conductivity in solid sodium chloride.

Ion Movement in Solids

The behavior of ions within solids like sodium chloride is crucial for understanding conductivity properties. In a crystalline solid, ions are locked into place by the strong electrostatic forces discussed earlier. This means that, unlike in a liquid or solution where ions can freely move, the ions in solids are restricted to vibrating around their fixed positions in the lattice.

The lack of movement means that the ions cannot carry electric charge across the solid, which is essential for electrical conductivity. While these ions might seem inactive, they are indeed in dynamic equilibrium within their positions, vibrating in response to temperature and other factors, albeit not enough to break free.

• Higher temperatures can lead to greater vibrational energy, but the ions still remain within the lattice.
• In order to conduct electricity, the ions must be liberated to move, which is why melting sodium chloride into a liquid can allow for conductivity.

Electrical Conductivity in Crystals

Understanding electrical conductivity in crystals like sodium chloride involves considering the movement of charge carriers. In the solid state, the ions are immobile, adhering tightly to the lattice structure without the freedom to move. This lack of movement means there are no mobile charged particles to facilitate the conduction of electricity.

For conductivity, it's not just the presence of ions that matters, but their ability to move freely. In electrolytes or metals, free ions or electrons can travel freely, allowing for electric currents to pass through. However, in crystalline solids, the immobile nature of ions impedes such movement.

• This explains the poor conductivity of solid sodium chloride.
• When melted or dissolved in water, sodium chloride dissociates into its ions, allowing them to move freely and conduct electricity efficiently.