Question

How will you distinguish between the following pairs of terms?
(i)Tetrahedral and octahedral sites
(ii)Crystal lattice and unit cell

Answer 1

Hint: The atoms in a solid are arranged in a particular fashion. Arrangement of atoms in an infinite and regular manner is called close packing of equal spheres can also be known as crystal lattice. Tetrahedral and octahedral sites are the voids in closely packed spheres.

Complete step by step solution:
(i)Tetrahedral and octahedral sites in close packing are occupied by atoms and ions in a crystal structure of salts and alloys. By recognising these sites, it helps in crystal studies.

Tetrahedral sites	Octahedral sites
The vacant site among 4 spheres in a tetrahedral arrangement is called tetrahedral sites.	The site or void formed by the equilateral triangles with apices in opposite directions is called octahedral void or site.
In a tetrahedral arrangement there are two tetrahedral sites for each sphere	In case of octahedral arrangement; each sphere has only one octahedral site.

(ii)

Crystal Lattice	Unit cell
The uniform three dimensional structural arrangements of atoms, ions and molecules in a crystalline solid is called crystal lattice. These atoms, ions and molecules are considered as points in space.	Unit cells are a small portion of the crystal lattice.
It is a set of uniformly ordered unit cells.	Unit cell is a basic unit of a crystal lattice.

There are different types of unit cell, primitive unit cell and centred unit cells. When the constituent particles occupy only the corners, then it is called a primitive unit cell. When the constituent particles occupy the corners as well as face, body or edge centres, then its called centred unit cell.

Note: Unit cells should not be confused with an atom or any other particle. It is the atoms which constitute a unit cell. If there are n number of spheres in a crystal lattice, then the number of tetrahedral voids will be 2n and the number of octahedral voids will be n.