Q: Which of the following exists as covalent crystals in the solid state ?

Explanation:

Covalent Crystals

Covalent crystals are solids where the atoms are bonded together by strong covalent bonds, forming a continuous network that extends throughout the material. This structure gives covalent crystals their characteristic hardness, high melting points, and stability.

Silicon exists as a covalent crystal in the solid state. In its crystalline form, silicon atoms are each bonded to four other silicon atoms in a tetrahedral arrangement, creating a robust and rigid three-dimensional network. This extensive bonding is typical of covalent crystals and is responsible for silicon’s semiconductor properties, making it essential in the electronics industry.

The other options do not form covalent crystals in the solid state:

– Iodine exists as molecular crystals, where I2 molecules are held together by weaker van der Waals forces rather than strong covalent bonds.
– Sulphur typically forms molecular crystals consisting of S8 rings, again held together by weaker intermolecular forces.
– Phosphorus can exist in several allotropes, but common forms like white phosphorus consist of molecular structures rather than a covalent network.

Therefore, among the given options, silicon is the one that exists as a covalent crystal in the solid state.