Question

The distance between \(\mathrm{Li}^{+}\) and \(\mathrm{Cl}^{-}\) is \(257 \mathrm{pm}\) in solid \(\mathrm{LiCl}\) and \(203 \mathrm{pm}\) in an \(\mathrm{LiCl}\) unit in the gas phase. Explain the difference in the bond lengths.

Short answer

The ionic bond in solid LiCl is longer due to interactions with neighboring ions in the crystal lattice, whereas in the gas phase, LiCl is a discrete molecule with a shorter bond length.

Step-by-step solution

Understand the Problem

We need to explain the difference in bond lengths between \(\mathrm{LiCl}\) in solid state and gas phase. The given bond lengths are 257 pm and 203 pm, respectively.

Identify the Nature of the Bonds

In solid \(\mathrm{LiCl}\), the compound forms a lattice structure. In this state, interactions involve not only the ionic bond but also other neighboring atoms, potentially elongating the bond. In contrast, in the gas phase, \(\mathrm{LiCl}\) is isolated, without interference from other atoms.

Consider the Effect of Lattice

In the solid state, \(\mathrm{LiCl}\) ions are part of a crystal lattice. The ionic bonds are influenced by mutual electrostatic attractions and repulsions experienced by neighboring ions in the lattice, often stretching the bonds compared to when the molecule is isolated.

Analyze the Bond Length in Gas Phase

In the gas phase, \(\mathrm{LiCl}\) exists as discrete molecules. There is no external electrostatic forces from neighboring ions, so the bond length is shorter, reflecting the inherent strength of the ionic bond without external long-range interactions.

Conclude

The difference in bond lengths is due to the interaction with surrounding ions in the solid lattice, which elongates the bond compared to the situation in the gas phase where the molecule is isolated and only internal forces determine the bond length.

Key concepts

Ionic Bonding

Ionic bonding is a type of chemical bond that occurs when atoms transfer electrons between each other. This results in the formation of positively charged ions, known as cations, and negatively charged ions, known as anions.
\(\text{Li}^+\) and \(\text{Cl}^-\) form such an ionic bond. Li, lithium, often loses one electron to become \(\text{Li}^+\), while Cl, chlorine, gains that electron to become \(\text{Cl}^-\). This charge difference creates a strong electrostatic attraction between the two ions.
The ionic bond strength influences various properties of the substance, including melting point, solubility, and electrical conductivity. In our case with \(\text{LiCl}\), its behavior changes between solid and gas phases due to the nature of the ionic bonding.

• Ions are significantly attracted, creating a very stable connection.
• It involves a complete transfer of electrons, as opposed to sharing in covalent bonds.
• The ionic bond is non-directional, meaning it equally influences bonds with any neighboring ions.

Crystal Lattice Structure

A crystal lattice structure is a highly ordered, repeating pattern of ions in a solid. In \(\text{LiCl}\), each lithium ion \((\text{Li}^+\)]) is surrounded by several chloride ions \((\text{Cl}^-\)]) and vice versa, forming a three-dimensional grid where each ion is held together by ionic bonds. This long-range order makes the solid very stable.
In solid form, these interactions are maximized because each ion is surrounded by oppositely charged ions in every direction. This has several consequences:

• The structure is extremely stable and rigid.
• It often results in high melting and boiling points due to strong inter-ionic attractions.
• When a force is applied, it can cause like-charged ions to realign, leading to brittleness.
• The ionic distances in the lattice can lead to larger inter-ionic spaces compared to isolated molecules.

Because of these reasons, solid \(\text{LiCl}\) shows different bonding characteristics compared to its gas phase. The lattice forces can "stretch" the bond lengths slightly due to constant electrostatic interference and accommodation.

Bond Length

Bond length is a crucial concept in chemistry, particularly when analyzing molecular and ionic compounds. It defines the distance between the centers of two bonded atoms or ions. In \(\text{LiCl}\), two different bond lengths were observed: 257 pm in the solid state and 203 pm in the gas phase.
The bond length can vary due to several factors:

• The presence of surrounding ions affects the bond in a crystal lattice structure.
• In the solid state, the multiple electrostatic interactions between neighboring ions cause the bonds to elongate.
• In the gas phase, the separation from the lattice enables ions to move closer, reflecting a shorter bond length.

It is essential to understand that bond lengths are not fixed but rather flexible, influenced by the physical state of the compound and the interactions with surrounding molecules or ions. By recognizing these influencing factors, students can better comprehend how bond lengths differ in varying environments.