Problem 1 - Chemistry: Introducing Inorganic, Organic and Physical Chemistry

Question

From the properties in the table below, describe whether each of the elements shown exists as a molecular structure, covalent network structure, or a metal. (Section 6.0) $$\begin{array}{lrrl} & T_{m} / \mathrm{K} & T_{b} / \mathrm{K} & \text { Conductivity } \\ \text { Boron } & 2348 & 4270 & \text { Insulator } \\ \text { Phosphorus (white) } & 317 & 554 & \text { Insulator } \\ \text { Lead } & 601 & 2022 & \text { Conductor } \end{array}$$

Step-by-Step Solution

Answer

Boron is a covalent network; phosphorus is molecular; lead is metallic.

Step 1: Understanding Molecular Structures

Molecular structures are typically associated with low melting and boiling points and are insulators or poor conductors. Elements that form simple covalent molecules exhibit this type of structure.

Step 2: Understanding Covalent Network Structures

Covalent network structures are characterized by very high melting and boiling points. They are usually insulators because their electrons are involved in covalent bonding, leaving no free electrons for conducting electricity.

Step 3: Understanding Metallic Structures

Metallic structures have variable melting and boiling points but are distinguished by their good electrical conductivity. This conductivity arises from the free movement of electrons in the metallic lattice.

Step 4: Analyzing Boron's Properties

Boron has a high melting point (2348 K) and boiling point (4270 K), and it is an insulator. These properties are indicative of a covalent network structure.

Step 5: Analyzing Phosphorus's Properties

White phosphorus has low melting (317 K) and boiling (554 K) points and is an insulator. These traits align with the characteristics of a molecular structure.

Step 6: Analyzing Lead's Properties

Lead has moderate melting (601 K) and boiling (2022 K) points and is a good conductor of electricity. These properties are typical of a metal.

Key Concepts

Molecular Structures

Molecular structures are fascinating yet quite straightforward in terms of their characteristics. They mainly form when elements combine to create simple covalent molecules. These structures are known to have:

• Low melting points
• Low boiling points
• Poor conductivity

\(\text{White phosphorus}\), with its relatively low melting (317 K) and boiling (554 K) points and non-conductive nature, is a perfect example of molecular structure. Insulators are common in molecular structures because there are no free electrons or ions to carry an electrical current. Therefore, even though they may seem simple, understanding these properties helps us identify molecular structures very easily.

Covalent Network Structures

Covalent network structures stand out due to their extremely robust and extensive bonding. Unlike molecular structures, these compounds manifest for elements that form an intricate web of covalent bonds. You will find them to have:

• Exceptionally high melting points
• Exceptionally high boiling points
• Generally insulative properties

The high melting point (2348 K) and boiling point (4270 K) of \(\text{Boron}\) coupled with its insulative behavior, make it a classic example of a covalent network structure. This is because, in these networks, all electrons are tightly bound within the covalent bonds, rendering them non-conductive. This deeply intertwined network is what gives them strength and stability.

Metallic Structures

Metallic structures are both unique and versatile, easily recognized due to their signature properties of conductivity and structural characteristics. Metals are marked by:

• Variable melting points
• Variable boiling points
• Excellent electrical conductivity

\(\text{Lead}\), for instance, possesses moderate melting (601 K) and boiling (2022 K) points. However, it is its outstanding conductivity that genuinely defines it as having a metallic structure. This conductivity arises from the sea of electrons that are free to move throughout the metal lattice, enabling easy transference of electrical charge. This makes metallic structures exemplary in scenarios requiring efficient conduction of electricity.

Electrical Conductivity

Electrical conductivity refers to the ability of a material to conduct an electric current. It significantly varies across different types of chemical structures due to the differences in electron availability.

• In molecular structures, like white phosphorus, the lack of free electrons results in poor conductivity.
• Covalent network structures, such as boron, generally do not conduct electricity well, since electrons are engaged in stable covalent bonds.
• Metallic structures, exemplified by lead, display high conductivity. This is due to the free movement of electrons within their structure, acting like a charged particle sea.

Understanding the relationship between these structures and conductivity helps in tailoring material properties for specific applications. It's these distinct characteristics that play critical roles in determining a material's usage in various electrical and industrial applications.