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Void Meaning

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The word "void" refers to gaps between constituent particles. In a densely packed structure, voids refer to the empty 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 or 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.


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 empty space created by joining the centers of these spheres forms a tetrahedral void.

Octahedral Voids: Octahedral voids are located next to tetrahedral voids.  We get a void created by enclosing six spheres when the triangular voids of the first layer correlate with the triangular voids of the layer above or below it. Octahedral Voids refer to the empty space created by combining the triangular voids of the first and second layers.


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

  • The octahedral void be N

  • The tetrahedral void be 2N

Difference Between Tetrahedral and Octahedral Voids

Tetrahedral Void

Octahedral Void 

The void is surrounded by four spheres. Hence the coordination number of the tetrahedral void is 4.

The void is surrounded by 6 spheres. Hence the coordination number of the octahedral void is 6.

In a tetrahedral void, the atom is surrounded by 4 atoms placed at the four corners of the tetrahedron.

In an octahedral void, the atom is surrounded by 6 atoms placed at the six corners of the octahedron.

The void is formed when a triangular void made up of coplanar atoms collides with the fourth atom above it or below it.

The void is formed when two sets of equilateral triangles pointing in opposite directions with six spheres.

The volume of the void is much smaller than spherical particles.

The volume of the void is small.

If R is the radius of a spherical particle, then the radius of the tetrahedral void is 0.225R

If R is the radius of a spherical particle, then the radius of octahedral l void is 0.414R

If the number of the closed packed spheres is N, then the tetrahedral void is 2N.

If the number of the closed packed spheres is N, then the octahedral void is N.


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 atoms, 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

FAQ (Frequently Asked Questions)

1. How Tetrahedral Voids are Formed?

Ans: The atom in the tetrahedral void is in contact with four atoms at each of the tetrahedron's four corners. When a triangular void made of coplanar atoms (first layer) comes into contact with the fourth atom above or below it, this void is formed (second layer).

2. Where Do You Find Octahedral Voids?

Ans: The body core of this unit cell has one octahedral void. Aside from the body center, each of the 12 edges has one of the octahedral voids in the center, which is surrounded by six atoms, four from the same unit cell and two from two neighboring unit cells.

3. Is it Possible For the Void Ratio to Be Less Than One?

Ans: The void ratio is the proportion of voids (open spaces, such as air and water) to solids in soil. As a consequence, the void ratio can be greater than one. It's even possible to express it as a fraction. The only difference between void ratio and porosity is in the denominator.