NCERT Solutions for Class 12 Chemistry Solid State – Types of Voids in Solids

NCERT Solutions for Class 12 Chemistry Solid State Types of Voids in Solids

Class 12 Chemistry Solid State: Voids literally mean gaps between the constituent particles. We define the void as the unoccupied empty space in a unit cell or empty space in the unit cell. Voids are also known as holes in the unit cell. Voids in solid states mean the vacant space between the constituent particles in a closely 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. In the case of hexagonal packing, these voids are in triangular shapes and are known as the triangular voids/ trigonal voids.

The void enclosed by three spheres/ particles in contact is called triangular voids/ trigonal voids. These voids and the spheres surrounding it are in the same plane. In this type of void, only a small sphere of radius 0.155 times of the bigger sphere can fit. Thus, in ionic crystals, the cation will occupy the trigonal voids enclosed by anions.

In three dimensional closed packing of constituent particle shows the following two types of voids:

1. Tetrahedral Voids:

A tetrahedral void is formed when one sphere or particle is placed in the depression formed by three particles. The vacant space or void among the four constituent particles having a tetrahedral arrangement in the crystal lattice makes a tetrahedral void.

Characteristics of tetrahedral voids

• The vacant space or void is surrounded by four atomic spheres. Hence co-ordination number of the tetrahedral void is 4.
• The atom in the tetrahedral void is in contact with four atoms placed at four corners of a tetrahedron.
• This void is formed when a triangular void made coplanar atoms is in contact with the fourth atom above or below it.
• The volume of the void is much smaller than that of the spherical particle.
• If R is the radius of the constituent spherical particle, then the radius of the tetrahedral void is 0.225 R.
• If the number of close-packed spheres is N, then the number of tetrahedral voids is 2N.

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2. Octahedral Void:

An octahedral void is formed when three close-packed spheres forming an equilateral triangle, are placed over another set of three spheres, in opposite directions. The vacant space or void at the center of six spheres or atoms forms an octahedral void.

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Characteristics of Octahedral Voids:

• The vacant space or void is surrounded by six atomic spheres. Hence, the coordination number of the tetrahedral void is 6.
• 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 pointing in the opposite direction with six
• The volume of the void is small.
• If R is the radius of the constituent spherical particle, then the radius of the octahedral void is 0.414 R.
• If the number of close-packed spheres is N, then the number of octahedral voids is N.

Covering Tetrahedral Voids:

Tetrahedral voids of the second layer may be covered by the spheres of the third layer. In this case, the spheres of the third layer are exactly aligned with those of the first layer. Thus, the pattern of spheres is repeated in alternate layers. This pattern is often written as ABAB ……. pattern. This structure is called a hexagonal close-packed (hcp) structure.

This sort of arrangement of atoms is found in many metals like magnesium and zinc.

Covering Octahedral Voids:

The third layer may be placed above the second layer in a manner such that its spheres cover the octahedral voids. When placed in this manner, the spheres of the third layer are not aligned with those of either the first or the second layer. This arrangement is called a ‘C’ type. Only when the fourth layer is placed, its spheres are aligned with those of the first layer as shown. This pattern of layers is often written as ABCABC ……….. This structure is called the cubic close-packed (ccp) or face-centered cubic (fcc) structure. Metals such as copper and silver crystallize in this structure.

Both these types of close packing are highly efficient and 74% space in the crystal is filled. Only 26% is in the form of voids.

In either of them, each sphere is in contact with twelve spheres. Thus, the coordination number is 12 in either of these two structures.

For more NCERT Solutions for Class 12 Chemistry click below…

Solid states

Classification Of Crystalline Solid

Unit Cell And Space Lattices

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