Question

How will you distinguish between the following pairs of terms: Crystal lattice and unit cell.

Answer 1

Hint: We need to know some properties of solid state and accordingly define crystal lattice and unit cell and distinguish between the two. Solids are a type of matter and have a definite mass, shape and volume. Their constituent particles (atoms, molecules or ions) have a fixed arrangement and order. Depending on the nature of order they are classified as crystalline or amorphous. The terms Crystal lattice and unit cell are restricted to crystalline solids only.

Complete answer:
We have to know that the crystalline solids have a definite characteristic geometrical shape due to a large number of small crystals present which also have a definite shape and order. There exists a regular pattern of arrangement of particles which repeats itself periodically over the entire crystal. Sodium chloride is an example of crystalline solid.
Let’s we discuss about the crystal lattice as,
-Crystal lattice: The main characteristic of crystalline solids is a regular and repeating pattern of constituent particles. If the three dimensional arrangement of constituent particles in a crystal is represented diagrammatically, in which each particle is depicted as a point, the arrangement is called crystal lattice. A portion of crystal lattice of sodium chloride is given below:

The red and green points in the image above represent the sodium and chloride ions. Hence it can be said that a regular three dimensional arrangement of points in space is known as crystal lattice and each point in a lattice is called a lattice point or lattice site which represents one constituent particle.
-Unit cell: In a crystal lattice, the smallest portion which when repeated in different directions generates the entire lattice is called the unit cell. The dimensions of a unit cell with respect to its constituents are mutually perpendicular and characterized by six sides or parameters as shown in the image above.

Note:
We must know that the crystal lattices are available in 14 different possible three dimensional lattices called Bravais Lattices. Unit cells are divided into two categories – primitive and centered unit cells. When the constituent particles are present only on the corner positions then it is called a primitive unit cell. The example given above is an example of primitive type. When one or more constituent particles are present in other positions in addition to those at corners are called centered unit cells.