How will you distinguish between the following pairs of terms:
1. Hexagonal close-packing and cubic close-packing?
2. Crystal lattice and unit cell?
3. Tetrahedral void and octahedral void?

Hint: An ordered arrangement of atoms, ions or molecules, which has occurred from the intrinsic nature of the constituent particles to form symmetric patterns that repeat themselves along all the principal directions of three - dimensional space in matter, is known as the crystal structure.

Complete step by step answer:
1. The hexagonal close packing $\left( {HCP} \right)$ and close cubic packing $\left( {CCP} \right)$ are the two forms of crystal structures. The difference between $HCP$ and \[CCP\] is that, in the $HCP$ crystal structure, third layer of spheres has the same arrangement of spheres as in the first layer and the second layer of spheres cover the tetrahedral voids of first layer. This arrangement is also known as ABAB….. whereas in the \[CCP\] crystal systems, the second layer of spheres do cover the half of the depressions caused by the first layer and third layer, which is completely different from the first and second layer, gets stacked in the depressions of the second layer. Also, the number of members per unit cell are \[6\] and \[4\] respectively in case of $HCP$ and \[CCP\] crystal structures.

2. The regular three dimensional arrangement of identical points of the ions, atoms and molecules in space is known as crystal lattice. Whereas, the unit cell is the smallest repeating portion of the crystal lattice which when repeated over and again in different directions produces the complete crystal lattice

3. A tetrahedral void is a simple triangular void in a crystal and is surrounded by four spheres which are arranged tetrahedrally around it. While, the octahedral void is a double triangular void having one vertex of triangle vertex facing downwards. The octahedral void is surrounded by six spheres.



Note:
The significance of the crystal structure is that it does give information about how the spheres of atoms, ions or molecules are being stacked together in the structure as it does help us to understand the intrinsic nature of the crystal or solid.