Problem 14 - Chemistry: The Central Science

Question

Indicate the type of solid (molecular, metallic, ionic, or covalent-network) for each compound: (a) InAs, (b) \(\mathrm{MgO}\), (c) \(\mathrm{HgS}\), (d) In, (e) HBr.

Step-by-Step Solution

Answer

The types of solids for each compound are: (a) InAs - covalent-network solid, (b) MgO - ionic solid, (c) HgS - ionic solid, (d) In - metallic solid, and (e) HBr - molecular solid.

Step 1: (a) InAs

InAs is formed by Indium (In) and Arsenic (As). Indium is a metal and Arsenic is a metalloid. When a metal bonds with a non-metal or a metalloid, it forms a covalent bond. Therefore, InAs is a covalent-network solid.

Step 2: (b) MgO

MgO is formed by Magnesium (Mg) and Oxygen (O). Magnesium is a metal and Oxygen is a non-metal. When a metal bonds with a non-metal, it forms an ionic bond. Therefore, MgO is an ionic solid.

Step 3: (c) HgS

HgS is formed by Mercury (Hg) and Sulfur (S). Mercury is a metal and Sulfur is a non-metal. When a metal bonds with a non-metal, it forms an ionic bond. Therefore, HgS is an ionic solid.

Step 4: (d) In

In is a metal element. Metallic compounds contain a lattice of metal cations surrounded by a sea of delocalized electrons. Therefore, In is a metallic solid.

Step 5: (e) HBr

HBr is formed by hydrogen (H) and bromine (Br), both of which are non-metal elements. When two non-metals bond together, it forms a covalent bond. Also, HBr exists as discrete molecules with their own covalent bonds, making it a molecular solid.

Key Concepts

Covalent-Network Solids

Covalent-network solids are fascinating in the realm of materials science due to their unique bonding structure. These types of solids are characterized by an extended network of covalent bonds. Each atom in a covalent-network solid is bonded to several neighboring atoms, creating a vast and rigid three-dimensional network. This results in certain properties:

• High melting and boiling points: The strong covalent bonds throughout the material require significant energy to break.
• Hardness and strength: The interconnected bonding network makes these solids very tough.
• Poor electrical conductivity: The absence of free-moving electrons (as in metals) tends to render these solids non-conductive.

Examples of covalent-network solids include diamond, silicon carbide ( ext{SiC}), and ext{InAs} as seen in the exercise. Their extensive network of bonds gives them exceptional durability and stability.

Ionic Solids

Ionic solids are formed when metals bond with non-metals through ionic bonds. This occurs when electrons are transferred from the metal to the non-metal, resulting in the formation of ions. The opposite charges on these ions create a strong electrostatic attraction, forming a solid. Key characteristics of ionic solids include:

• High melting and boiling points: The strong ionic attractions require a lot of energy to overcome.
• Brittleness: While strong, when stress is applied, the ions can shift and repel each other, leading to fractures.
• Electrical conductivity: They conduct electricity when melted or dissolved in water, as the ions are free to move and carry charge.

Examples include ext{MgO} and ext{HgS}, as highlighted in the exercise. Each displays the typical traits of ionic compounds due to their ionic bonding nature.

Metallic Solids

Metallic solids are primarily made up of metal atoms. What sets them apart is their unique structure, often described as a "sea of electrons." In this model, metal cations are surrounded by a sea of delocalized electrons, which allows these solids to possess some remarkable properties:

• Electrical conductivity: The free-flowing electrons can easily carry the electric current.
• Luster: The electron sea reflects light, giving metals their characteristic shine.
• Malleability and ductility: Metals can be hammered into sheets or drawn into wires without breaking due to the mobility of the electrons.

An example as indicated in the exercise is brilliant Indium (In), showing these distinct physical properties that make it useful in a variety of applications.

Molecular Solids

Molecular solids consist of simple or complex molecules held together by weak forces like van der Waals interactions, hydrogen bonds, or dipole-dipole interactions. This type of bonding results in specific properties that differentiate them from other solids:

• Low melting and boiling points: The weak intermolecular forces are easier to overcome compared to covalent or ionic bonds.
• Softer textures: The weak forces do not create a rigid structure, making them generally softer.
• Poor electrical conductivity: Molecules in these solids do not have free electrons to conduct electricity.

Example: ext{HBr} as described in the exercise. This compound forms through the covalent bonding of hydrogen and bromine into discrete molecules, which pack together in the solid-state, but interact through weaker forces, embodying the characteristics of a molecular solid.