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Void

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Last updated date: 19th Apr 2024
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Void Meaning

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

  • The octahedral void be N

  • The tetrahedral void be 2N


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

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 point 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.


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

FAQs on Void

1. How are Tetrahedral Voids formed?

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?

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?

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.

4. What are voids in crystals?

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.

5. What is a triangular void?

The triangular voids are found in the planes of the close-packed structures, whenever three spheres are in contact in such a fashion. Join the centers of three spheres, an equilateral triangle ABC is formed. The space seen in the plane of the triangle is called interstitial triangular void.

6. How are tetrahedral and octahedral voids formed?

When the triangular voids of the first layer are covered by spheres of the next layer, tetrahedral voids are formed. A tetrahedral void is surrounded by four spheres. The overlapping triangular voids from the two layers together form an octahedral void. An octahedral void is surrounded by six spheres.

7. What is the relation between tetrahedral void and octahedral void?

As you see that tetrahedral voids are triangular. When two such voids combine, from two different layers they form an octahedral void. So when the tetrahedral void of the first layer and the tetrahedral void of the second layer align together, they form an octahedral void.

8. What is the shape of the octahedral void?

The atom in the octahedral void is in contact with six atoms placed at six corners of an octahedron. This void is formed when two sets of equilateral triangles point in the opposite direction with six spheres. The volume of the void is small.

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