Question

Which of the following statements is falsc? (1) \(\mathrm{SiC}\) and \(\mathrm{B}_{4} \mathrm{C}\) are covalent carbides. (2) SiC is widely uscd as an abrasive called corrundum. (3) SiC has a 3D structure of \(\mathrm{Si}\) and \(\mathrm{C}\) atoms cach tetrahcdrally surrounded by four atoms of other kind. (4) \(\mathrm{B}_{4} \mathrm{C}\) is harder than \(\mathrm{SiC}\) and used as an abrasive and also as a shicld from radiation.

Short answer

The false statement is: SiC is widely used as an abrasive called corrundum.

Step-by-step solution

Understand the types of carbides

Covalent carbides are formed when carbon atoms bond covalently with atoms of other elements. \(\text{SiC}\) (Silicon Carbide) and \(\text{B}_4\text{C}\) (Boron Carbide) are examples of covalent carbides because the bonds between the carbon atoms and the silicon or boron atoms are covalent.

Verify the abrasive use of SiC

SiC is indeed used as an abrasive. However, it is not called corrundum. Corrundum refers to \(\text{Al}_2\text{O}_3\), not \(\text{SiC}\). The correct name for \(\text{SiC}\) when used as an abrasive is carborundum.

Confirm the crystal structure of SiC

SiC has a three-dimensional structure where each silicon atom is covalently bonded to four carbon atoms and vice versa, forming a tetrahedral arrangement.

Compare hardness and other properties

Boron carbide \(\text{B}_4\text{C}\) is indeed harder than silicon carbide \(\text{SiC}\). Besides being used as an abrasive, \(\text{B}_4\text{C}\) is also used as a shield from radiation.

Conclusion

Comparing all the statements, the incorrect one is that SiC is widely used as an abrasive called corrundum. The correct name when SiC is used as an abrasive is carborundum.

Key concepts

Silicon Carbide

Silicon carbide (SiC) is a compound made of silicon and carbon atoms arranged in a 1:1 ratio. It forms a crystalline structure where silicon atoms and carbon atoms are bonded together in a tetrahedral arrangement. This structure gives SiC its remarkable properties, such as high hardness and thermal conductivity. SiC is widely used in various applications, including semiconductors, high-temperature materials, and many industrial abrasives. It is most commonly known as carborundum, which is not to be confused with corundum, a form of alumina (Al2O3). Silicon carbide is highly resistant to heat and chemical attack, which makes it a preferred material in harsh environments.

Boron Carbide

Boron carbide (B4C) is an exceedingly hard material composed of boron and carbon atoms. It is one of the hardest known materials, second only to diamond and cubic boron nitride. Its hardness and low density make it ideal for applications requiring lightweight yet highly durable materials. Boron carbide is used extensively as an abrasive in polishing and lapping applications, as well as in cutting tools. Additionally, its properties are leveraged in armor plating and radiation shielding materials due to its effectiveness in absorbing neutrons. Boron carbide's unique combination of hardness, resistance to chemical attack, and lightweight makes it invaluable in these high-stress applications.

Abrasive Materials

Abrasive materials are substances used to wear down, smooth, or polish other softer materials through friction. Common abrasives include compounds like silicon carbide (SiC) and boron carbide (B4C). These materials are chosen for their high hardness and ability to retain cutting edges under extreme conditions. They are used in a variety of applications, ranging from sandpaper and grinding wheels to cutting discs and precision lapping tools. Abrasive materials play a crucial role in manufacturing processes to achieve fine finishes and precise tolerances on metal, ceramic, and glass surfaces.
Other commonly used abrasives include alumina (Al2O3), also known as corrundum, and diamond, the hardest natural material.

Covalent Bonds

Covalent bonds are chemical bonds formed when two atoms share one or more pairs of electrons. This type of bonding is characterized by the mutual sharing of electrons, which helps achieve a stable electron configuration for the atoms involved. In the case of covalent carbides like silicon carbide (SiC) and boron carbide (B4C), the carbon atoms form strong covalent bonds with silicon or boron atoms. These bonds create a very strong and stable lattice structure, contributing to the hardness and thermal stability of these materials.
The strength of covalent bonds can be attributed to the precise and permanent sharing of electron pairs, making materials like SiC and B4C extremely hard and durable.

Crystal Structure

The crystal structure of a material refers to the ordered arrangement of atoms, ions, or molecules in a solid. In materials like silicon carbide (SiC) and boron carbide (B4C), the crystal structure is critical to their unique properties. SiC possesses a tetrahedral structure where each silicon atom is surrounded by four carbon atoms and vice versa. This three-dimensional network of covalent bonds gives SiC its exceptional hardness and strength.
Boron carbide (B4C) has a more complex crystal structure, comprising 12-atom icosahedra of boron atoms linked by carbon atoms, creating a dense and robust lattice. Understanding the crystal structure is essential for predicting and explaining the mechanical, thermal, and electrical properties of these materials. The stability and arrangement of atoms in these structures make covalent carbides incredibly resilient and useful in demanding applications.