Problem 146 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following exists as covalent crystals in the solid state? (a) Sulphur (b) Phosphorus (c) Iodine (d) Silicon

Step-by-Step Solution

Answer

Silicon exists as covalent crystals in the solid state.

Step 1: Understanding Covalent Crystals

Covalent crystals are solid materials where atoms are bonded together in a continuous network by covalent bonds. To identify which substance forms covalent crystals, we need to know about the bonding and structure of each option.

Step 2: Analyze Sulphur

Sulphur exists in several allotropic forms, the most common being orthorhombic sulfur (S8) rings, which are held together by van der Waals forces in the solid state. These are not covalent crystals.

Step 3: Analyze Phosphorus

Phosphorus also has several allotropes, like white phosphorus (P4) which consists of molecular units held together in the solid state by van der Waals forces. It does not form a covalent crystal structure.

Step 4: Analyze Iodine

Iodine molecules (I2) are held together in their solid state by van der Waals forces, which means this structure is molecular and not a covalent crystal.

Step 5: Analyze Silicon

Silicon forms a network solid with a giant covalent structure where each silicon atom is covalently bonded to four other silicon atoms in a tetrahedral lattice. This is the typical structure of a covalent crystal.

Step 6: Conclusion

Among the given options, Silicon is the only element that forms covalent crystals in its solid state due to its network of covalent bonds forming a rigid structure.

Key Concepts

Covalent Bonding

Covalent bonding is a powerful type of chemical bonding where atoms share pairs of electrons to achieve stability. This kind of bond is strong and occurs between non-metal atoms. Such a durable bond arises because both atoms have a similar tendency to attract electrons to themselves, often leading to the formation of molecules with a stable electron configuration. In covalent bonding:

• Electrons are shared rather than transferred from one atom to another.
• This sharing allows each atom to reach the electron configuration of a noble gas, which is typically more stable.
• The bond strength can vary, but it is generally higher than that of ionic bonds.

Covalent bonds are critical in the formation of various substances such as gases, liquids, and certain solids, known as covalent crystals.

Allotropes

Allotropes are different forms of the same chemical element, with atoms bonding in distinct ways, leading to different structures and properties. For instance, carbon can exist as diamond and graphite, each showcasing diverse properties due to varied atomic arrangements. Key points about allotropes include:

• The same chemical element can form completely different structures.
• These structures, or allotropes, have unique physical and chemical properties.
• The differences arise from variations in bond geometry and atomic connectivity.

Allotropes highlight the versatility of elements like carbon, sulfur, and phosphorus, each exhibiting multiple physical forms with distinct bond angles and strengths influencing their functionality.

Network Solids

Network solids are unique structures where every atom is interconnected through covalent bonds, forming one large network. Such solids are known for their hardness and high melting points due to strong covalent bonding throughout the structured lattice. Network solids include compounds like diamond and silicon dioxide. Important characteristics are:

• Atoms are bonded in a continuous network throughout the material.
• These are typically very hard and have high melting points.
• No discrete molecules can be identified within the solid.

A classical example is silicon, which forms a tetrahedral network that grants it the properties of a true covalent crystal.

Crystal Structures

Crystal structures refer to the orderly and repeating arrangement of atoms in a solid. Such arrangements are determined by how atoms or molecules are packed together and influence the material's properties. A well-known structure is the cubic arrangement often seen in metals and salts. Key characteristics include:

• Crystal structures define the spatial arrangement of atoms within a material.
• They are classified by the symmetry and geometry of these arrangements.
• Different structures lead to varied physical properties like hardness and clearness.

In covalent crystals like diamond or silicon, the crystal structure plays a vital role in determining their mechanical strength and conductivity, highlighting the importance of internal atomic distribution.