Question

How many nearest neighbours are there in an atom or ion for an octahedral hole of a closed packed structure? (a) 4 (b) 6 (c) 8 (d) 12

Short answer

The number of nearest neighbours in an atom or ion for an octahedral hole of a closed packed structure is 6.

Step-by-step solution

Understand the Octahedral Hole

An octahedral hole in a closed packed structure is surrounded by atoms or ions at each of the six corners of an octahedron. This implies that an atom or ion in an octahedral hole is in contact with six nearest neighbors.

Count the Nearest Neighbours

Since the octahedral hole is defined by having six surrounding atoms or ions, the count of nearest neighbours is exactly six.

Identify the Correct Option

Based on the count of nearest neighbours for an octahedral hole in a closed packed structure, the correct answer is (b) 6.

Key concepts

Closed Packed Structure

In materials science, a closed packed structure represents the way atoms are densely packed together in a crystal. It maximizes the space efficiency, meaning there are as few gaps as possible between the atoms. The close-packed planes of atoms form layers where each atom is surrounded by neighbors in a regular pattern.

Imagine a floor covered entirely with equally sized spheres—that's a nice visualization of a closed packed arrangement. This specific arrangement can be of two types, namely hexagonal close-packed (hcp) and face-centered cubic (fcc) structures. In both of these arrangements, the octahedral holes come into play. They are the spaces between the atoms that could theoretically accommodate another atom without changing the overall structure.

An atom in an octahedral hole is touched by six others, which leads us to another crucial concept: the coordination number. This neatly bridges our discussion into understanding how each atom relates to its neighbors.

Coordination Number

The coordination number of an atom in a crystal structure is defined as the number of other atoms to which it is immediately adjacent. For instance, in the context of an octahedral hole in a closed packed structure, the coordination number is six. This is because there are six atoms or ions that make up the corners of the octahedron, each one in contact with the atom in the center of the octahedral hole.

The coordination number provides information about the strength and directionality of the bonding between atoms and plays a vital role in determining the physical properties of the material. For example, a higher coordination number typically indicates that an atom is more strongly bonded to its neighbors and may have a higher density and melting point. This concept of coordination number is key to understanding various properties of materials and their behavior under different conditions.

In addition to the octahedral holes, there are also tetrahedral holes in close-packed structures, but these have a coordination number of four as they are surrounded by four atoms.

Crystal Lattice

The crystal lattice is a highly ordered and symmetric arrangement of atoms or ions that extends in all three spatial dimensions. Think of it as a scaffold that organizes the pattern in which atoms are laid out in a material. Every point on a crystal lattice, called a lattice point, has an environment that is exactly identical to every other point.

The crystal lattice can take on several shapes, such as cubic, tetragonal, or hexagonal, each with different attributes and symmetries. These shapes are the key determinants of the crystal's physical properties, such as cleavage, symmetry, and even its electrical and heat conductivity.

---

Importance of Lattice in Solutions

Understanding the crystal lattice is also crucial in solving problems related to material science and solid-state physics. For example, when discussing an octahedral hole within a closed packed structure, we refer to the specific location in the lattice where the atom or ion is placed. Knowledge of the crystal lattice concept helps students visualize and comprehend more complex topics such as crystal growth, defects, and diffusion.