Question

Classify each of these solids as ionic, metallic, molecular, network, or amorphous. (a) \(\mathrm{KF}\) (b) \(\mathrm{I}_{2}\) (c) \(\mathrm{SiO}_{2}\) (d) \(\mathrm{BN}\)

Short answer

(a) Ionic, (b) Molecular, (c) Network, (d) Network

Step-by-step solution

Classify KF

The compound \( \text{KF} \) is a combination of a metal (potassium, \( \text{K} \)) and a non-metal (fluorine, \( \text{F} \)). In such compounds, the metal loses electrons to the non-metal, resulting in the formation of ions. Therefore, \( \text{KF} \) is classified as an **ionic** solid.

Classify I₂

\( \text{I}_2 \) consists of non-metal iodine atoms bonded together by covalent bonds. These molecules interact via weak Van der Waals forces when in solid form, making this a molecule-based solid. Thus, \( \text{I}_2 \) is classified as a **molecular** solid.

Classify SiO₂

\( \text{SiO}_2 \), or silicon dioxide, is known for its covalent bonds forming a continuous network. Each silicon atom is covalently bonded to four oxygen atoms in a three-dimensional framework. This type of structure is characteristic of a **network** solid.

Classify BN

\( \text{BN} \), or boron nitride, has alternating boron and nitrogen atoms bonded together in a manner similar to that of graphite in one polymorph, or diamond for another. Due to its continuous strong covalent bonding network, \( \text{BN} \) is classified as a **network** solid.

Key concepts

Ionic Solids

Ionic solids are one of the fundamental types of solids. They are formed from the electrostatic attraction between positively charged cations and negatively charged anions. For instance, in potassium fluoride (KF), potassium acts as a cation (K\(^+\)) and fluorine acts as an anion (F\(^-\)). These ions form a repeating pattern, called a lattice, which is the backbone of the solid structure.
Key characteristics include:

• High melting points due to strong ionic bonds.
• Usually hard and brittle.
• They conduct electricity when melted or dissolved in water, because the ions are free to move.

These properties are exhibited by many common salts, which makes ionic solids crucial in various applications, including chemical and industrial processes.

Molecular Solids

Molecular solids are created by molecules held together by relatively weak forces such as Van der Waals interactions or hydrogen bonds. In the case of iodine (I\(_2\)), it forms a solid through these weak forces between the iodine molecules.
Features of molecular solids include:

• Generally have low melting and boiling points due to weak intermolecular forces.
• These solids are often soft and have varying solubility in different solvents.
• Do not conduct electricity because they lack ions or free electrons.

These characteristics make molecular solids ideal for use where low-heat stability and non-conductivity are needed, such as in certain pharmaceuticals and organic compounds.

Network Solids

Network solids are a fascinating type of solid characterized by a continuous network of covalent bonds. Silicon dioxide (SiO\(_2\)) and boron nitride (BN) are classic examples. In SiO\(_2\), each silicon atom bonds covalently to four oxygen atoms, forming an extensive three-dimensional lattice. Similarly, BN forms either a structure akin to graphite or diamond, depending on the form.
Noteworthy properties of network solids include:

• High melting points and boiling points due to strong covalent bonds throughout the network.
• Very hard and typically poor conductors of electricity.
• Insoluble in most solvents.

Network solids find applications in instances where materials must withstand extreme conditions, like abrasives, tools, and electronics.

Amorphous Solids

Amorphous solids do not exhibit the ordered, regular pattern seen in other types of solids like ionic, molecular, or network solids. Instead, these solids have disordered atomic or molecular structures. Common examples include glass and plastic.
Characteristics of amorphous solids are:

• Do not have a definite melting point. Instead, they soften over a range of temperatures.
• Typically, they may have varied mechanical properties, such as flexibility or brittleness.
• Random arrangement leads to isotropy, meaning their properties are the same in all directions.

Amorphous solids are particularly useful in applications like packaging and insulating materials, where non-crystalline structure is advantageous.

Metallic Solids

Metallic solids are formed by metal atoms sharing a "sea of electrons." These electrons are delocalized, which means they are not bound to any particular atom, allowing them to move freely throughout the entire structure. This electron cloud model gives metallic solids their unique properties.
Key characteristics of metallic solids include:

• Good electrical and thermal conductivity due to free-moving electrons.
• Typically malleable and ductile, allowing them to be shaped without breaking.
• Reflective or shiny appearance because they absorb and emit light over a broad spectrum.

These properties make metallic solids essential for numerous applications, especially in construction and electrical industries.