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Chemistry notes for class 12-Solid State-Types of Voids in Solids Voids are also known as holes in the unit cell.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. 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 dime