Concept: Electrical Conductivity in Ionic Solids

Electrical conductivity in ionic compounds depends on the presence of mobile charge carriers. In the solid state, ionic compounds form a crystal lattice where ions are held in fixed positions and cannot move freely. For electricity to be conducted, there must be free-moving charged particles (ions or electrons).

Why (C) is correct:

Solid NaCl is an ionic compound consisting of Na${}^{+}$ and Cl${}^{-}$ ions. In the solid state, these ions are rigidly held in the crystal lattice and are not free to move. Therefore, there is no velocity of ions, which are the charge carriers in ionic compounds, preventing the flow of electricity.

Option Analysis:

• A) In solid NaCl there are no ions: This is incorrect. Solid NaCl is composed of Na${}^{+}$ and Cl${}^{-}$ ions arranged in a crystal lattice.
• B) Solid NaCl is covalent: This is incorrect. NaCl is a classic example of an ionic compound, formed by the electrostatic attraction between Na${}^{+}$ and Cl${}^{-}$ ions.
• D) In solid NaCl there are no electrons: This is incorrect. While electrons are not the primary charge carriers in ionic solids (ions are), the atoms themselves contain electrons. However, these electrons are localized within the ions and are not delocalized or free to move like in metals.

Correct Answer: (C)