## Shaillee Kaushal is teaching live on Unacademy Plus

XII 01 SOLIDS II

Types of symmetry 1. Centre of symmetry: It is an imaginary point within the crystal that any line drawn through it intersects the surface of the crystal at equal distances in both directions. Only one centre of symmetry is present in a crystal. Centre of symmetry of a cubic crystal

2. Plane of symmetry: Imaginary plane which passes through the c of the crystal and divides it into two equal halves. NaCl possesses in all nine planes of symmetry: (i) three rectantular planes (ii) six diagonal planes. ntre 4 fold axis 3 fold axis 2 fold axis

3. Axis of symmetry: It is an imaginary straight line about which if a crystal is rotated it will present the same appearance more than once during complete revolution. Diad -if the original appearance is repeated twice. (after an angle of ) Triad thrice (after angle of 120 degrees) Tetrad four times (after an angle of 90 degrees) Hexad-six times (after an angle of 60 degrees) NaCl has 13 axis of symmetry Axis of two fold syety Axis of threef old symety

Elements of symmetry: Total of 23 elements of symmetry are present. Plane of symmetry (3+6)-9 Axes of symmetry (3+4+6)-1.3 Centre of symmetry 1 Total - 23