Question

\(\mathrm{NaCl}\) crystal is (a) FCC (b) \(\mathrm{BCC}\) (c) HCP (d) simple Cubic

Short answer

NaCl crystal is (a) FCC.

Step-by-step solution

Identify the Crystal Structure of NaCl

To solve which crystal structure (sodium chloride) NaCl belong to, recall the arrangement of ions in its unit cell. NaCl crystallizes in a face-centered cubic (FCC) structure where the Na and Cl ions alternate to create a three-dimensional repeating pattern.

Eliminate Incorrect Options

Based on the known crystal structure of NaCl, eliminate the incorrect options. NaCl does not form in a body-centered cubic (BCC) structure, hexagonal close-packed (HCP) structure, or a simple cubic structure.

Select the Correct Crystal Structure

Since NaCl forms in a face-centered cubic (FCC) structure, option (a) FCC is the correct answer.

Key concepts

Face-Centered Cubic (FCC) Structure

The face-centered cubic (FCC) structure, one of the primary types of crystal systems found in chemistry, is pivotal in understanding materials such as sodium chloride, commonly known as table salt. In an FCC lattice, atoms are positioned at each of the eight corners of a cube and the centers of all the cube faces. This arrangement creates a highly dense and compact structure.

Imagine placing a single atom at each corner of a cube and then one directly in the center of each face—now, you have the basic snapshot of an FCC unit cell. The fascination with the FCC structure comes from its efficiency in filling space, allowing more atoms to be packed into a given volume than other arrangements.

Unique Properties:

• Each corner atom is shared among eight adjacent cells, and each face-centered atom is shared with two cells.
• The coordination number, which is the number of nearest-neighbor atoms, is 12 for FCC structures, signifying that each atom contacts 12 others.
• When it comes to NaCl, the structure is slightly altered because it contains two different ions, Na⁺ and Cl⁻, arranged such that each ion is surrounded by six of the opposite kind, maintaining electrical neutrality.

This tightly-packed structure has wide implications in material sciences, particularly concerning the strength and durability of metals and ionic solids.

Crystal Systems in Chemistry

Crystals, the beautifully ordered structures that they are, come in various shapes and sizes that can be categorized into seven crystal systems in chemistry. Each system is defined by its unique symmetries and axial lengths and angles, which result in distinctive unit cells. We encounter these systems in nature and in the substances used in our everyday lives.

Here are the seven crystal systems:

• Cubic (also known as isometric)
• Tetragonal
• Orthorhombic
• Hexagonal
• Trigonal or Rhombohedral
• Monoclinic
• Triclinic

Each crystal system has one or more lattice types associated with it, such as primitive cubic, body-centered cubic (BCC), and face-centered cubic (FCC) within the cubic system. These systems and their corresponding lattice types play a crucial role in determining the physical properties of materials.

For instance, NaCl adopts the FCC type within the cubic system due to the regular and optimal arrangement of ions. Understanding the specifics of these systems not only supports academics but also aids in the development of new materials with desired characteristics for various applications.

Unit Cell Arrangements

A unit cell is the smallest repeating unit in a crystal lattice that shows the full symmetry of the entire crystal. It's like a building block, repeating itself in three dimensions to create the larger structure of a crystal. There are several types of unit cells, each with its characteristics and geometries that dictate the overall structure and properties of a crystal.

In chemistry and materials science, recognizing these arrangements is important for predicting how a substance will interact with its environment, including its melting point, density, and how it conducts electricity or heat. The major unit cell types are:

• Primitive (P): Atoms only at the cell corners.
• Body-Centered (I): Atoms at all cell corners and one at the center of the cell.
• Face-Centered (F): Atoms at all cell corners and at the center of each face, like that found in the NaCl crystal structure.

Understanding these arrangements is critical in fields ranging from materials engineering to pharmacology because the unit cell serves as a fundamental predictor of a material's attributes. The FCC structure of NaCl ensures that each sodium ion is equidistant from six chloride ions in a three-dimensional matrix, which is central to its stability and solubility.