Which of the following exists as covalent crystal in the solid state?

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Engineering

Chemistry

Classification of Solids

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Question

Which of the following exists as covalent crystal in the solid state?

A

Sulphur

B

Iodine

C

Silicon

D

Phosphorus

Si exists as covalent crystal solid, while sulphur (S₈), phosphorus (P₄), Iodine (I₂) exist as molecular crystal solid.

A covalent crystal (or network covalent solid) is a type of solid where atoms are bonded together in a continuous network by covalent bonds. These crystals are characterized by high melting points, extreme hardness, and poor electrical conductivity (unless doped). They are typically formed by elements from Group 14 of the periodic table, like Carbon (diamond), Silicon, and Germanium.

Let's analyze the given options:

Sulphur (S): Exists as S₈ molecules held together by weak van der Waals forces. It is a molecular solid, not a covalent crystal.

Iodine (I₂): Exists as diatomic molecules held together by van der Waals forces. It is also a molecular solid.

Silicon (Si): Has a diamond cubic crystal structure, where each silicon atom is covalently bonded to four other silicon atoms in a tetrahedral arrangement. This forms a giant covalent network, making it a covalent crystal.

Phosphorus (P): Commonly exists as P₄ molecules (white phosphorus) or in polymeric forms (red phosphorus) with molecules held by intermolecular forces. It is a molecular solid.

Therefore, among the options, Silicon exists as a covalent crystal in the solid state.

Related Topics and Formulae

Types of Solids:

Covalent Crystals: Atoms linked by covalent bonds (e.g., diamond, silicon, quartz).
Molecular Solids: Molecules held by van der Waals forces, dipole-dipole interactions, or hydrogen bonds (e.g., iodine, sulphur, ice).
Ionic Solids: Ions held by electrostatic forces (e.g., NaCl, KCl).
Metallic Solids: Metal atoms with delocalized electrons (e.g., Fe, Cu).

Key Properties of Covalent Crystals:

Very high melting and boiling points.
Extreme hardness.
Poor electrical conductivity (insulators), except semiconductors like silicon.
Insolubility in common solvents.

Structure: The diamond cubic structure for silicon and carbon (diamond) can be described with each atom having a coordination number of 4, forming a tetrahedral geometry. The unit cell is face-centered cubic with atoms at positions (0,0,0), (0,½,½), (½,0,½), (½,½,0) and interior positions at (¼,¼,¼), (¼,¾,¾), (¾,¼,¾), (¾,¾,¼).