Which type of solid is \(SiO_2\)?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Covalent Solid
In quartz (\( \ce{SiO2} \)) every silicon atom is bonded covalently to four oxygen atoms and every oxygen to two silicon atoms, forming a giant three-dimensional network of Si–O covalent bonds. There are no separate molecules or ions; the constituent particles are atoms held by covalent bonds. Such solids are called covalent or network solids; they are very hard, have very high melting points and are insulators.
Why the other options are incorrect
Option (1) Molecular Solid — Incorrect. Molecular solids (ice, solid \( \ce{CO2} \), \( \ce{I2} \)) consist of discrete molecules held by weak intermolecular forces and melt at low temperatures; \( \ce{SiO2} \) has no discrete molecules.
Option (2) Ionic Solid — Incorrect. Ionic solids (NaCl) are made of ions held by electrostatic attraction; Si–O bonds are covalent, not ionic.
Option (3) Metallic Solid — Incorrect. Metallic solids have positive ions in a sea of delocalised electrons and conduct electricity; quartz is an insulator.
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 – “… (4) Covalent or network solids — Constituent particles: Atoms; Bonding/attractive forces: Covalent bonding; Examples: SiO2 (quartz), SiC, C (diamond), AlN, C (graphite); Physical nature: Hard; Electrical conductivity: Insulators (graphite: conductor, exception); Melting point: Very high”