Problem 70 - Chemistry: The Central Science

Question

(a) Silicon is the fundamental component of integrated circuits. Si has the same structure as diamond. Is \(\mathrm{Si}\) a molecular, metallic, ionic, or covalent-network solid? (b) Silica is \(\mathrm{SiO}_{2}\). What type of solid would you expect silica to form?

Step-by-Step Solution

Answer

(a) Silicon (Si) is a covalent-network solid due to its diamond-like structure and the formation of a strong lattice network by the covalent bonding of its four valence electrons. (b) Silica (SiO₂) forms a covalent-network solid as well, as it consists of a strong 3D network of covalent bonds between silicon and oxygen atoms.

Step 1: Identifying the solid type for Silicon (Si)

Silicon has the same structure as diamond, which is a covalent-network solid. In a covalent-network solid, the atoms are bonded together by a network of covalent bonds. This bond results in a large and very strong lattice structure. Silicon also has four valence electrons, like carbon in diamond, which can participate in covalent bonding. Therefore, we can conclude that silicon (Si) is a covalent-network solid.

Step 2: Identifying the solid type for Silica (SiO₂)

Silica is a compound of silicon and oxygen and has the chemical formula SiO₂. In a silica molecule, one silicon atom is covalently bonded to two oxygen atoms. The oxygen atoms are also covalently attached to other silicon atoms in the lattice structure. This results in a strong 3D network of covalent bonds that hold the atoms together, similar to the covalent network solid structure found in silicon. Therefore, we can conclude that silica (SiO₂) forms a covalent-network solid as well.

Key Concepts

Silicon

Silicon is a crucial element in modern technology, primarily in the field of electronics as the key component of integrated circuits. It is known for having the same crystal structure as diamond, known as the diamond cubic structure. Silicon is classified as a covalent-network solid. This means that the atoms in silicon are bonded through a continuous network of covalent bonds, forming a strong lattice structure. Each silicon atom shares electrons with four neighboring silicon atoms, creating a tetrahedral arrangement. This structure is both extremely stable and strong due to the rigidity of the covalent bonds. Additionally, silicon, similar to carbon in a diamond, has four valence electrons, making it ideally suited for forming stable covalent connections throughout the lattice.

Diamond Structure

The diamond structure is a unique type of cubic crystal structure. It is characterized by a repeating pattern where each atom is covalently bonded to four other atoms, creating a tetrahedral geometry. This arrangement gives rise to the distinctive properties of substances such as diamond and silicon:

• High Hardness and Strength: The strong covalent bonds make the structure very hard and durable.
• Stability: The three-dimensional network resists external strain, contributing to its stability.
• Electrical Insulation: Due to the lack of free-moving electrons, the diamond structure generally acts as an insulator.

In materials like silicon, the diamond structure is essential for its functionality in semiconductor devices as it ensures the right balance between electrical conductivity and resistance.

Covalent Bonding

Covalent bonding is a form of chemical bonding characterized by the sharing of electron pairs between atoms. This type of bonding results in the formation of molecules and networks that are immensely strong and stable. Key attributes of covalent bonding include:

• Directional Nature: The bonds form in specific directions, leading to complex and precise crystal structures.
• High Melting and Boiling Points: The substantial energy required to break covalent bonds results in materials with high thermal stability.
• Electrical Insulation: The absence of free electrons implies that covalent materials typically do not conduct electricity.

In network solids like silicon and silica, covalent bonding sustains their robust lattice structures, essential for their usage in various technological applications.

Silica

Silica, with the chemical formula \( \text{SiO}_2 \), is another example of a covalent-network solid. It consists of silicon and oxygen atoms, where each silicon atom is bonded to two oxygen atoms. The oxygen atoms, in turn, connect to other silicon atoms, forming an extensive three-dimensional lattice. This structure contributes to several distinctive properties of silica:

• High Thermal Stability: The strong bonds make silica able to withstand high temperatures without degradation.
• Hardness: Similar to diamond and silicon, silica is hard and resistant to physical abrasion.
• Insulation Capabilities: Silica acts as an excellent electric insulator, owing to no free electrons in its structure.

Silica's persistent use as a material in glassmaking, ceramics, and electronics derives from these remarkable attributes, underpinned by its robust network of covalent bonds.