Question

Arsenic tribromide melts at \(31.1{ }^{\circ} \mathrm{C}\), diffracts X rays, and does not conduct electricity in either the solid or liquid phase. What type of solid is it: amorphous, ionic, molecular, covalent network, or metallic?

Short answer

Arsenic tribromide is a molecular solid.

Step-by-step solution

Identify Physical Properties

Start by reviewing the given physical properties: Arsenic tribromide melts at 31.1°C, it diffracts X rays, and does not conduct electricity in either the solid or liquid phase.

Analyze Melting Point

Consider the melting point of 31.1°C. This relatively low melting point suggests that arsenic tribromide might be a molecular solid, as these typically have lower melting points compared to ionic or covalent network solids.

Examine Electrical Conductivity

Observe that arsenic tribromide does not conduct electricity in either phase (solid or liquid). This eliminates ionic solids, which generally conduct electricity when molten, and metallic solids, which conduct in both solid and liquid forms.

Consider X-ray Diffraction

Since arsenic tribromide diffracts X rays, it must have a well-organized, repeating crystal structure, indicating that it is a crystalline solid and not an amorphous one.

Final Conclusion

Given that arsenic tribromide has a low melting point, lacks electrical conductivity, and has a crystalline structure, the most likely classification for this solid is molecular.

Key concepts

Arsenic Tribromide

Arsenic tribromide is a chemical compound with the formula \( \text{AsBr}_3 \). In its solid state, arsenic tribromide is a reasonably soft solid that melts at a modest temperature of 31.1°C.
This melting point suggests that it may not involve strong ionic or covalent bonds like those found in metals or covalent network solids. Instead, arsenic tribromide exists as discrete molecules held together by weaker intermolecular forces.
Due to its properties, it's categorized as a molecular solid. This means that arsenic tribromide does not engage in electrical conduction, given that its composition prevents electron flow in both solid and liquid phases. The electrons in arsenic tribromide are more involved in molecular bonding rather than creating pathways for conductivity.

Properties of Molecular Solids

Molecular solids are a type of solid composed of molecules held together by intermolecular forces, such as van der Waals interactions, dipole-dipole forces, or hydrogen bonds. These forces are much weaker than ionic or covalent bonds, which is why molecular solids have distinct characteristics:

• Low Melting and Boiling Points: Because of the weaker forces holding them together, molecular solids melt and boil at lower temperatures compared to other types of solids.
• Poor Electrical Conductivity: In molecular solids, there are no free ions or electrons available to conduct electricity.
• Solubility in Polar and Nonpolar Solvents: The solubility of these solids depends on the nature of their constituent molecules and the type of forces between them.
• Crumbliness: Typically, these solids are soft or brittle, owing to their relatively weak intermolecular forces.

Molecular solids, like arsenic tribromide, exhibit these properties, distinguishing them from ionic, metallic, or covalent network solids.

Electrical Conductivity in Solids

Electrical conductivity in solids refers to the ability of a substance to allow the flow of electric current. This depends on the presence of charge carriers, like ions or free electrons. Different types of solids have varied conductivity based on their structure and bond types:

• Ionic Solids: Conduct electricity when they are dissolved in water or melted, as their ions become free to move.
• Metallic Solids: Conduct electricity in both solid and liquid states due to delocalized electrons that create a "sea" of electrons.
• Covalent Network Solids: Generally poor conductors, as electrons are localized in bonds.
• Molecular Solids: Like arsenic tribromide, typically do not conduct electricity in any state, owing to their lack of free-moving charge carriers.

Understanding these distinctions is crucial in determining the type of solid based on its conductivity characteristics.

Crystalline Structure in Chemistry

The term crystalline structure refers to a highly ordered arrangement of atoms, ions, or molecules within a solid. This structure is characterized by a repeating pattern that extends in all three spatial dimensions, providing both stability and unique properties to the material. X-ray diffraction is a key method used to identify the crystalline nature of a substance.
In the case of arsenic tribromide, its ability to diffract X-rays confirms its crystalline nature.
Crystalline solids can be classified as molecular, ionic, metallic, or covalent network solids, each possessing a distinct type of bonding and arrangement. This crystalline nature leads to distinct characteristics, such as:

• Definite Geometric Shape: Crystals have well-defined surfaces and angles.
• Sharp Melting Points: Unlike amorphous solids, which melt over a range, crystalline solids tend to have specific melting temperatures.
• Anisotropy: Crystals may exhibit different physical properties when measured in different directions.

These properties help in identifying and categorizing a solid base on its internal structure.