Question

Sodium chloride has a crystalline structure made up of \(\mathrm{Na}^{+}\)and \(\mathrm{Cl}^{-}\)ions. Why does \(\mathrm{NaCl}\) not conduct electricity in solid state? (a) Solids do not conduct electricity. (b) The ions of \(\mathrm{NaCl}\) become mobile only in molten state and are not free to move in solid state. (c) The crystalline structure does not have ions. (d) When a bond is formed between ions they lose their charges.

Short answer

The correct answer is (b) because the ions in solid NaCl are not mobile and can only move when NaCl is in molten state.

Step-by-step solution

Understanding Electrical Conductivity

Electrical conductivity in a substance primarily depends on the mobility of charged particles. An electrical current is conducted when these charged particles are free to move in response to an external electric field.

Evaluating the Options

Analyze the given options (a) to (d) considering the nature of sodium chloride (NaCl) and the requirements for electrical conductivity.

Option (a) Evaluation

The claim that solids do not conduct electricity is incorrect as some solids, like metals, do conduct electricity due to the movement of free electrons.

Option (b) Evaluation

In a solid state, the ions in the NaCl crystalline lattice are held in place by strong electrostatic forces and do not have the freedom to move. In a molten state, these forces are sufficiently overcome, allowing the ions to become mobile and conduct electricity.

Option (c) Evaluation

This option is incorrect because the crystalline structure of NaCl contains a large number of ions, which is fundamental to its formation.

Option (d) Evaluation

This option is incorrect because the formation of ionic bonds between Na+ and Cl- ions does not result in them losing their charges; it results in the establishment of a stable ionic lattice.

Key concepts

NaCl Crystalline Structure

Sodium chloride, commonly known as table salt, forms a distinctive crystalline lattice. This structure is an arrangement of sodium (Na+) and chloride (Cl-) ions organized in a repeating pattern that spans three dimensions. The stability of this lattice stems from the electrostatic attraction between the positively charged sodium ions and the negatively charged chloride ions. This arrangement creates what is known as a face-centered cubic lattice, where each sodium ion is surrounded by six chloride ions and vice versa.

Understanding the NaCl crystalline structure is key in grasping why, even though it is made up of ions, it doesn't conduct electricity in its solid form. The ions are locked in place, rendered immobile by the strong ionic bonds. In this state, they cannot move to conduct an electrical current. It is only when NaCl is melted into a liquid or dissolved in water that these rigid bonds are broken, freeing the ions to move within the liquid and therefore conduct electricity.

Mobility of Ions

The concept of ion mobility is central to understanding electrical conductivity. In the context of sodium chloride, the ions are only able to conduct electricity when they can move freely.

In solid NaCl, the ions are fixed within their lattice points and are not free to move about. Without this mobility, there is no flow of charge, and thus no electrical conductivity. When NaCl is melted or dissolved, the ionic lattice breaks down. The ions are then no longer held in place and can move in response to an electric field – a principle that is paramount in the functionality of batteries and electrochemical cells.

Ionic Bonds

At the heart of the NaCl crystalline structure are the ionic bonds – a type of chemical bond formed through the electrostatic attraction between oppositely charged ions. NaCl is created when sodium (Na), which has a single electron in its outer shell, transfers that electron to chlorine (Cl), which needs an extra electron to complete its shell.

The result is a sodium ion with a positive charge and a chloride ion with a negative charge. It's a common misconception that ions lose their charges when they form bonds; instead, they achieve full outer electron shells, which significantly lowers their potential energy and creates a strong bond. The nature of ionic bonding explains why the ions in solid NaCl do not have the freedom to move – they are held in place by these incredibly strong attractions, known as ionic bonds.