Solids have definite mass, volume and shape. Depending upon order, they are classified as crystalline solids and amorphous solids. Crystalline solids are further classified into molecular, ionic, metallic and covalent solids. Crystalline solids are made up of constituent particles which are arranged in a regular pattern, which is depicted in the form of three dimensional array of points in spaces called lattice points. Small characteristic repeating portion of lattice is called unit cell.
Unit cells can be primitive and also centred unit cells like face centred, body centred, end centred. Solids are not perfect. There are various types of defects (i) line defect (ii) point defect. Point defects are of three types : stoichiometric, impurity and non-stoichiometric defects. Solids show magnetic properties depending upon the orientation of electron.
Based on the paragraph Answer the following :
CaF2 is an example of :
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Ionic solid
In ionic solids the constituent particles are ions, held in a three dimensional arrangement by strong coulombic (electrostatic) forces. \( \ce{CaF2} \) is made of Ca2+ and F− ions (the fluorite structure); like NaCl and MgO it is hard and brittle, has a high melting point, and conducts electricity only when molten or dissolved.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Classification of Crystalline Solids – …
