Question

On the basis of the description given, classify each of these solids as molecular, metallic, ionic, network, or amorphous, and explain your reasoning. (a) A brittle, yellow solid that melts at \(113^{\circ} \mathrm{C} ;\) neither the solid nor the liquid conducts electricity (b) A soft, silvery solid that melts at \(40^{\circ} \mathrm{C} ;\) both the solid and the liquid conduct electricity (c) A hard, colorless, crystalline solid that melts at \(1713^{\circ} \mathrm{C} ;\) neither the solid nor the liquid conducts electricity (d) A soft, slippery solid that melts at \(63^{\circ} \mathrm{C} ;\) neither the solid nor the liquid conducts electricity

Short answer

(a) Molecular, (b) Metallic, (c) Network, (d) Molecular.

Step-by-step solution

Analyzing Substance (a)

The substance described is a brittle, yellow solid that melts at 113°C, and neither the solid nor the liquid conducts electricity. The brittleness, yellow color, and lack of electrical conductivity suggest that it is likely composed of non-metallic elements. The moderate melting point indicates molecular forces. Therefore, it is likely a molecular solid.

Analyzing Substance (b)

The substance described is a soft, silvery solid that melts at 40°C, and both the solid and liquid conduct electricity. The description of softness, metallic luster (silvery), and conductivity in both forms clearly suggests the characteristics of a metallic solid.

Analyzing Substance (c)

The substance described is a hard, colorless, crystalline solid that melts at 1713°C, and neither the solid nor the liquid conducts electricity. The high melting point and crystalline nature are indicative of strong covalent bonds in a network solid, such as diamond or quartz, which do not conduct electricity.

Analyzing Substance (d)

The substance described is a soft, slippery solid that melts at 63°C, and neither the solid nor the liquid conducts electricity. The soft, slippery nature and low melting point suggest the presence of weak van der Waals forces typical in molecular solids, such as paraffin wax.

Key concepts

Molecular Solids

These solids are composed of small molecules held together by weak forces such as van der Waals forces, dipole-dipole interactions, or hydrogen bonds. As a result, molecular solids generally have lower melting and boiling points compared to other solid types. They do not conduct electricity in solid or liquid form because they lack free-moving charged particles.
For example, the substance described as a brittle, yellow solid that melts at 113°C is likely a molecular solid because of its non-conductivity and moderate melting point. Common examples of molecular solids include substances like iodine and sugar.

• Typically soft and low-melting.
• Non-conductive due to absence of charges.
• Forces holding them together are intermolecular, not ionic or covalent within a network.

Metallic Solids

These solids are composed of metal atoms that share a 'sea' of delocalized electrons. This electron sea allows metallic solids to conduct electricity and heat efficiently. The metals' atoms are arranged in an orderly pattern, which gives metallic solids their characteristic luster and malleability.
An example provided is a soft, silvery solid that melts at 40°C, both in its solid and liquid forms, conducts electricity—a clear indicator of a metallic solid. Metals like copper and aluminum are representative of this class.

• Electrical and thermal conductivity due to free electrons.
• Malleable and ductile.
• Wide range of melting points.

Network Solids

Network solids, also known as covalent network solids, consist of atoms bonded by covalent bonds in a continuous network extending throughout the material. This results in high melting points, hardness, and often brittleness. They typically do not conduct electricity, as there are no free-moving charged particles.
An example is a hard, colorless, crystalline solid with a high melting point of 1713°C, which neither in solid nor liquid form conducts electricity. This indicates a network solid, like diamond or quartz.

• High melting point due to strong covalent bonds.
• Typically hard and brittle.
• Non-conductive, insulating.

Ionic Solids

These materials are made up of positive and negative ions held together by strong ionic bonds. As a result, ionic solids are typically hard and brittle with high melting points. They do not conduct electricity in their solid state due to the fixed position of ions, but they are good conductors when melted or dissolved in water, as ions become mobile.
Classic examples include table salt (NaCl) and calcium oxide (CaO).

• High melting and boiling points.
• Non-conductive as solids but conductive in molten or dissolved state.
• Generally hard and brittle.

Amorphous Solids

Amorphous solids lack a long-range order or repeating pattern of atoms as found in crystalline solids. Instead, their particles are arranged randomly, which gives them distinctive properties. Unlike crystalline solids, amorphous ones do not have a sharp melting point; they soften gradually when heated.
These solids can include glass and certain plastics, showing unique behaviors often used in various applications.

• No definite shape or crystal structure.
• Soften over a range of temperatures.
• Can be manipulated and molded easily when softened.