Question

Which of the following exists in polymeric form? (a) \(\mathrm{B}_{2} \mathrm{H}_{6}\) (b) \(\mathrm{AlCl}_{3}\) (c) \(\mathrm{BeCl}_{2}\) (d) \(\mathrm{SiC}\)

Short answer

(c) BeCl_{2} and (d) SiC exist in polymeric form.

Step-by-step solution

Understanding Polymeric Forms

Polymeric forms refer to a structure where a compound forms extensive chains or networks by repeating its basic unit. This often occurs due to the nature of the bonding between atoms, where they can form additional bonds with other units, creating a larger network.

Examine Option (a) _{2} H_{6}

Diborane (_{2} H_{6}) is known to exist as a discrete molecule and does not readily form polymeric structures. Its structure is characterized by two bridging hydrogen atoms, creating a special type of bond known as a three-center two-electron bond.

Examine Option (b) AlCl_{3}

Aluminum chloride (AlCl_{3}) can exist in both monomeric and polymeric forms. At elevated temperatures, it exists as a monomer, but under standard conditions, it often forms a polymeric network through dative bonds between aluminum and chlorine atoms.

Examine Option (c) BeCl_{2}

Beryllium chloride (BeCl_{2}) commonly forms a polymeric structure. In the solid state, it forms an infinite chain structure due to the overlapping of electron orbitals, resulting in extended bonding along the chain.

Examine Option (d) SiC

Silicon carbide (SiC) forms a giant covalent network. It is a hard, crystalline material where silicon and carbon atoms are bonded in a polymeric, lattice structure extending in three dimensions.

Identify Polymer Formation

Both BeCl_{2} and SiC exist in polymeric forms. BeCl_{2} forms polymeric chains, and SiC forms a giant covalent network structure. Therefore, both (c) and (d) exhibit polymeric forms.

Key concepts

Boron Hydrides

Boron hydrides, specifically diborane (\(\mathrm{B}_2\mathrm{H}_6\)), are fascinating compounds. These compounds consist of boron and hydrogen and are known for their unique bonding. Unlike many other hydrides, diborane forms through a special type of bonding called a three-center two-electron bond. This bond involves two boron atoms and two bridging hydrogen atoms, creating a stable but non-polymeric structure.
Although diborane forms discrete molecules, it does not extend into a polymeric form. The lack of additional bonding sites prevents the formation of extensive networks or chains typical of polymers. Instead, diborane remains a small, discrete unit with its unique atomic arrangement.

Aluminum Chloride

Aluminum chloride (\(\mathrm{AlCl}_3\)) is notable for its ability to exist in both monomeric and polymeric forms. In its monomeric form, typically seen at high temperatures, the compound consists of simple molecular units. However, under standard conditions, aluminum chloride assumes a polymeric form.
The polymeric structure arises because each aluminum atom can form coordinate bonds with chloride ions. This dative bonding allows the formation of an extensive network, with multiple aluminum and chlorine atoms linked together. This network results in a solid structure, characteristic of many polymeric compounds. Aluminum chloride's flexibility in forming both monomeric and polymeric forms makes it an essential study in polymer chemistry.

Beryllium Chloride

Beryllium chloride (\(\mathrm{BeCl}_2\)) is a classic example of a compound existing in a polymeric form. In its solid state, the compound forms an infinite chain structure, facilitated by electron orbital overlap between beryllium and chlorine atoms. These overlapping orbitals allow for continuous bonding along a chain, contributing to its stability.
This polymeric chain structure contrasts with a simple monomeric form where individual molecules do not link extensively. The polymeric formation in beryllium chloride is driven by the need for fulfilling electronic configurations, resulting in strong extended bonds. Such characteristics highlight the unique behaviors of metal halides in forming polymeric compounds.

Silicon Carbide

Silicon carbide (\(\mathrm{SiC}\)), also known as carborundum, is a prominent example of a giant covalent network structure. Unlike typical polymers, silicon carbide forms a robust crystalline lattice composed of silicon and carbon atoms. This robust lattice structure extends in three dimensions, creating a hard and durable material.
The strength and stability of silicon carbide arise from strong covalent bonds between silicon and carbon atoms, resulting in exceptional thermal and mechanical properties. These properties make silicon carbide an ideal material for high-performance applications, including abrasives and semiconductors. Its polymeric nature is evident in its repeating structural motif, contributing to its distinctive characteristics.

Polymer Chemistry

Polymer chemistry focuses on studying compounds formed by polymerization processes, where small units, or monomers, connect to form larger structures known as polymers. This field encompasses various materials, from simple plastic composites to complex biological molecules like proteins and DNA.
Understanding how monomers link to form polymers is crucial in polymer chemistry. Different bonding types, such as covalent bonds and coordinate bonds, play key roles in determining a polymer's properties. This knowledge enables the synthesis of materials with specific traits, like flexibility, strength, or chemical resistance. By exploiting various bonding possibilities, chemists can design polymers for diverse applications.

Molecular Structures

Molecular structures refer to the specific arrangement of atoms within a molecule, determined by the type of bonds and their spatial orientation. Understanding molecular structures is foundational in predicting chemical behavior and properties. These structures range from simple linear forms to complex 3D networks, influencing a substance's reactivity, stability, and functionality.
In polymer chemistry, specifically, molecular structures dictate the polymer's characteristics. For instance, the difference between a linear polymer and a cross-linked network significantly affects physical properties. By manipulating molecular structures, chemists can tailor materials to meet specific requirements, aiding in technological and industrial advancements.