[Chemistry Class Notes] on Void Pdf for Exam

The word “void” refers to gaps between constituent particles. In a densely packed structure, voids refer to the space between constituent particles (voids in chemistry). Solids can be packaged in one of three ways: one-dimensional (1D), two-dimensional (2D), or three-dimensional (3D).

When atoms are arranged in square close packing of hexagonal close packing, we see empty spaces between them in 2-dimensional structures.

These empty spaces are known as voids, and in hexagonal packing, these voids have triangular shapes and are referred to as triangular voids. Thus, the vacant spaces in a closely packed arrangement are called voids.

Tetrahedral and Octahedral Voids

In hexagonal packing, these triangular voids are seen in two different orientations. The apex of the triangle in one row points upward, while the apex of the triangle in the other row points downward. 

In the three-dimensional structure, about 26% of total space is empty and not occupied by spheres in both CCP and HCP near packing in solids. Interstitial voids, interstices, or gaps are the names given to these empty spaces. The above voids in solids are proportional to the number of spheres present. 

In a three-dimensional structure, there are two types of interstitial voids:

Tetrahedral Voids: In a cubic close-packed structure, the second layer’s spheres are above the first layer’s triangular voids. Each sphere touches the first layer’s three spheres. It forms a tetrahedron by joining the centers of these four spheres, and the space created by joining the centers of these spheres forms a tetrahedral void. In a closed packed structure, the number of tetrahedral voids is two times the number of spheres. Let the number of spheres be n. Then the number of tetrahedral voids will be 2n.

Octahedral Voids:  Adjacent to tetrahedral voids you can find octahedral voids. Octahedral voids are located next to tetrahedral voids. So now, what are Octahedral Voids? When the triangular voids of the first layer coincide with the triangular voids of the layer above or below it, we get a void that is formed by enclosing six spheres. This vacant space formed by combining the triangular voids of the first layer and that of the second layer is called Octahedral Voids. Octahedral Voids refer to the space created by combining the triangular voids of the first and second layers. If the number of spheres in a close-packed structure is n, then the number of octahedral voids will be n.

Number of Voids

The number of these two types of voids depends on the number of closed-packed spheres.

If the number of closed packed spheres is N, then

What is the Primary Difference between Tetrahedral and Octahedral Voids?

Tetrahedral voids are unoccupied empty spaces present in substances having a tetrahedral crystal system. Octahedral voids are unoccupied empty spaces present in substances having an octahedral crystal system. It can be found in substances having a tetrahedral arrangement in their crystal system. A tetrahedral void is a simple triangular void in a crystal and is surrounded by four spheres arranged tetrahedrally around it. On the other hand, an octahedral void is a double triangular void with one triangle vertex upwards and the other triangle vertex downwards and is surrounded by six spheres.

Difference between Tetrahedral and Octahedral Voids

In-depth Concept of Void

Voids mean gaps between the constituent particles. Voids in solid states mean the vacant space between the constituent particles in a closed-packed structure. Close packing in solids can be generally done in three ways: 1D close packing, 2D close packing, and 3D close packing.

In 2 dimensional structures when the atoms are arranged in square close packing and hexagonal close packing, we see empty spaces left over between the atoms. These empty spaces are called voids and in the case of hexagonal packing, these voids are in triangular shapes and are known as the triangular voids.

Did You Know?

The unit cell, or building block of a crystal, is the smallest repeating unit of the crystal lattice. 

The identical unit cells are described in such a way that they fill the available space without overlapping. A crystal lattice is a three-dimensional arrangement of atoms, molecules, or ions within a crystal. It comprises a large number of unit cells. Per lattice point is occupied by one of the three constituent particles.

Numerous unit cells together make a crystal lattice. Constituent particles like atom, molecules are also present. Each lattice point is occupied by one of these particles. 

Primitive Cubic Unit Cell 

1. Body-Centered Cubic Unit Cell 

2. Face Centered Cubic Unit Cell