Question

Monoclinic sulfur is an example of a monoclinic crystal system. What are the characteristics of the crystal system?
A.$a\ne b\ne c,\alpha =\beta =\gamma ={{90}^{\circ }}$
B.$a\ne b\ne c,\alpha \ne \beta \ne \gamma \ne {{90}^{\circ }}$
C.$a=b\ne c,\alpha =\beta =\gamma ={{90}^{\circ }}$
D.$a\ne b\ne c,\alpha =\gamma ={{90}^{\circ }},\beta \ne {{90}^{\circ }}$

Answer 1

Monoclinic is a type of crystal system in which the three axis are of unequal length. The two axes (a and c) are inclined towards each other at an oblique angle. The third axis (b) is perpendicular to the other two and is called the ortho axis. This crystal system forms a rectangular prism with parallelogram as its base. So, the two pairs of vectors are perpendicular and the third pair makes an angle other than${{90} ^ {\circ}}$.

Complete answer:
The crystal structures are classified according to the symmetry of their unit cell. There are a total seven crystal systems namely Triclinic, Monoclinic, Orthorhombic, Tetragonal, Trigonal, Hexagonal, and Cubic. There are seven crystal systems only mathematically it is impossible to create others in three-dimensional space as others are cut down to one of these only.
In the monoclinic crystal system the three axes are of unequal length and two of them are perpendicular to each other.

So, the correct choice is (D) as it shows the three axes a, b and c of unequal lengths and two sides being perpendicular to each other and the third one makes some other angle.

Additional Information:
In the cubic system the three axes are of equal lengths and intersect each other at right angles, ex- diamond.
In the tetragonal system the two axes are of equal length and they are in the same plane. The main axis is either longer or shorter, and all the three intersect at right angles, ex- zircon.
In a hexagonal system three out of four axes are in one plane, they are of the same length, and intersect each other at angles of ${{60}^{\circ }}$. The fourth axis is of a different length and intersects the others at right angles, ex- emerald.
In the trigonal system the axes and angles are similar to those in the Hexagonal System, and the two systems are often combined as a Hexagonal System. In the cross-section of a Hexagonal crystal, there will be six sides while in the cross-section of a Trigonal crystal there will be three sides, ex- quartz.
In an orthorhombic system the three axes are of different lengths and are at right angles to each other, ex- topaz.
In the triclinic system all the three axes are of different lengths and are inclined towards each other, ex- rhodonite.

Note:
The crystal structure is described by both geometry and atomic arrangements within the unit cell, whereas a crystal system is described only in terms of the unit cell geometry, ex- face-centered cubic is a crystal structure that belongs to the cubic crystal system.