Problem 17 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

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

Step 1: Understanding the structure of sodium chloride

Sodium chloride (NaCl) is an ionic compound composed of sodium ions (Na^+) and chloride ions (Cl^-) arranged in a crystal lattice structure. This structure is rigid and does not allow the ions to move freely within the solid.

Step 2: Analyzing conductivity

Electrical conductivity in a substance is possible if there are free-moving charged particles, such as ions or electrons. In solid NaCl, the ions are fixed in place in the crystal lattice and cannot move freely, meaning they cannot carry electric charge through the material.

Step 3: Eliminating incorrect options

Option (a) 'it contains free molecules' is incorrect as NaCl does not consist of molecules but ions. Option (c) 'it does not possess ions' is incorrect because NaCl is made of ions. Option (d) 'it contains only molecules' is incorrect for the same reason as option (a).

Step 4: Selecting the correct answer

The correct explanation for why crystalline sodium chloride is a bad conductor of electricity is that (b) the ions present in it are not free to move. In the solid state, the rigid arrangement of the crystal lattice keeps the ions fixed in place.

Key Concepts

Electrical Conductivity

Electrical conductivity is the ability of a substance to conduct electric current. This capability depends largely on the movement of charged particles. In metals, electrons are free to move and thus, they are excellent conductors. However, in ionic compounds like sodium chloride - particularly in its solid form - the charged particles are ions rather than electrons.
For electrical conduction to occur, these ions must be able to move. When ionic compounds are dissolved in water or melted, the ions become free to move, allowing the compound to conduct electricity.
This is not the case for solid NaCl, where ions are locked in place within the crystal lattice. Consequently, since there is no movement of ions in its solid state, sodium chloride does not conduct electricity until it is melted or dissolved.

Crystal Lattice Structure

The crystal lattice structure is a repeating arrangement of ions, atoms, or molecules in a crystalline material. Sodium chloride (NaCl) has a specific crystal lattice structure known as a face-centered cubic lattice.
This structure is characterized by - each sodium ion (Na⁺) being surrounded by six chloride ions (Cl⁻) - and vice versa. This 3D pattern repeats itself throughout the material, resulting in a highly stable and compact structure.
The crystal lattice is held together by strong electrostatic forces between the positively charged sodium ions and the negatively charged chloride ions. However, this rigid structure locks the ions in place, preventing them from moving freely.
This immobility in the lattice at room temperature is precisely why solid sodium chloride cannot conduct electricity.

Sodium Chloride

Sodium chloride, commonly known as table salt, is a classic example of an ionic compound. It is composed of equal numbers of sodium ions (Na⁺) and chloride ions (Cl⁻). These ions attract each other to form a strong, stable ionic bond.

• Ionic bonds result from the transfer of electrons between atoms, resulting in positively charged and negatively charged ions.
• This strong bond contributes to the high melting point and specific physical properties of NaCl, such as brittleness as well as solubility in water.

When sodium chloride dissolves in water, the ionic bonds are broken, and the individual ions are surrounded by water molecules.
This process, known as dissociation, allows the ions to move freely and conduct electricity in the solution. This illustrates the stark difference in the electrical conductivity behavior of NaCl: while a poor conductor in solid form, it becomes a good conductor when in solution.