[Physics Class Notes] on Orthorhombic System Pdf for Exam

This article contains an explanation of the orthorhombic system, examples, and some solved examples. If you incur any doubts while reading the article you can refer to the frequently asked questions at the end.

In the study of crystals, there are classifications based on the basic geometry of the crystal. Scientists named these groups the primitive crystal systems. The orthorhombic crystal system is one of the categories into which we can classify our crystalline solids. The names of all the seven crystal systems are as follows: cubic, tetragonal, orthorhombic, hexagonal, triclinic, monoclinic, and rhombohedral. These systems work on the principle of the three axes on which we draw the crystals and the relation of the axes to each other. This article lays focus on the orthorhombic structure. 

Orthorhombic Crystal

In this category of crystal systems, we can place the crystal on three mutually perpendicular axes which are of unequal length. This property is the defining characteristic of this group. The orthorhombic crystal system has its own set of unit cells. To understand what a unit cell is, one must imagine the atoms or atom groups in the crystal structure as points. If we join these points, we obtain a structure known as the lattice. If we observe, this lattice consists of stacks of blocks, and these we call unit cells. 

 

The orthorhombic is a three-axis System each of which are mutually at 90° angles and x axis is vertical and the shortest with the a axis which is intermediate in length and is from front to back and the B axis is taken as left to right with the longest.

Orthorhombic Unit Cell

The orthorhombic unit cell has its own set of unique characteristics. This unit cell has a set of three axes which we call the axes of twofold symmetry. If we rotate the crystal about these lines by an angle of 180°, the crystal will not change its appearance. To fulfill this property, the unit cell must have certain defined characteristics. In an orthorhombic unit cell, the angle between any two edges is always 90°. The edge lengths can, however, be unequal. 

 

Examples of Orthorhombic Structure 

Some examples of crystalline solids which take up the orthorhombic structure are as follows:

• enargite 

• marcasite 

• cerussite 

• barite 

• staurolite

• topaz

• orthoenstatite 

• aragonite 

• olivine 

• cementite 

• alpha-sulfur 

 

 

Variants of the Orthorhombic Crystal System

The orthorhombic system undergoes further classification to give rise to three subgroups of crystals. The common characteristic of these subgroups is that they consist of three mutually perpendicular axes which are unequal in length. These types are named the body-centred orthorhombic crystal system, base-centred orthorhombic crystal system, and face-centred orthorhombic crystal system respectively. The definitions of each of these crystal systems are as below: 

• Body-Centred Orthorhombic Crystal System: In this type of crystal structure, the lattice point is in the middle of the unit cell. 

• Base-Centred Orthorhombic Crystal System: In this type of crystal structure, there is a lattice point in the middle of each of the two ends. 

• Face-Centred Orthorhombic Crystal System: In this type of crystal structure, there exists a lattice point in the middle of each side. 

Solved Examples 

1. Define an Orthorhombic Crystal. 

Answer: Orthorhombic system is one of the structural categories of crystals systems into which we can classify crystalline solids. In this type of crystal structure, the lattice consists of three axes that are perpendicular to each other and whose lengths may or may not be inconsistent. 

 

2. State Whether this Statement is True or Not: In the Orthorhombic Crystal System, All Axial Angles are of the Same Value and are Identical. 

Answer: The statement is correct. The orthorhombic structure is such that all the crystallographic angles are of the same measure. This property is a fundamental characteristic of this type of crystal. 

 

3. State How a Crystal Structure and a Crystal System are Different Terminologies.

Answer: When we talk about a crystal structure, we mean the arrangement of atoms and the geometry which exists within the unit cell of the crystal. However, when talking of crystal systems, only the unit cell geometry comes into consideration. For example, body-centred and face-centred are crystal structures that fall under the orthorhombic crystal system.

 

4. Why is it So Essential to Know about Crystal Systems?

Answer: The importance of our awareness of crystal systems and crystal structures becomes clear when we obtain crystalline solids for use in our daily life. Two crystals having different structures will exhibit very different physical characteristics. The finest example of this is graphite and diamond. Due to their separate crystal systems and structures, one of these solids is exceptionally hard while the other is soft. These properties define their application, and thus, knowing how these crystals will behave under different physical conditions is essential.

 

5. What is orthorhombic and monoclinic?

Answer. Monoclinic is (crystallography) has three unequal axes with two perpendiculars and one oblique intersection while orthorhombic is (crystallography) has three unequal axes at right angles. Monoclinic and orthorhombic are two different kinds of crystal systems. Molecules or atoms in the case of a monoclinic crystal system are different to that of an orthorhombic Crystal system.

6. How many atoms are in orthorhombic?

Answer.  Although there are many crystal structures that fit with “orthorhombic” symmetry, the simple orthorhombic crystal structure has exactly 1 atom per lattice point in the simple orthorhombic Bravais lattice. Primitive orthorhombic crystals based on the Bravais lattice that exist with multiple atoms overall displaying primitive orthorhombic symmetry.

7. What is orthorhombic Sulphur?

Answer. Orthorhombic sulphur, Sα: The most significant form of sulphur is orthorhombic, Sα.  This is described as orthorhombic Sulphur,α Sulphur, Muthmann sulphur and rhombic sulphur by many. It is stable at room temperature and also at atmospheric pressure. Rhombic sulphur has a molecular structure. In the neighbouring molecules, the closest distance of approach of atoms is about 3.3A.