Atomic, Ionic, and Molecular Solids: Videos & Practice Problems

Solids are categorized as either crystalline or amorphous based on the arrangement of their constituent particles. Crystalline solids, such as ionic solids, molecular solids, covalent network solids, and metals, have highly ordered structures. Ionic solids, formed from cations and anions, are brittle, hard, and possess high melting points, exemplified by sodium chloride. Molecular solids, composed of molecules bonded by intermolecular forces, are generally softer and have lower melting points, like ice. Covalent network solids, linked by covalent bonds, are extremely hard with very high melting points; diamonds are a prime example. Metals, characterized by metallic bonds and a pool of electrons, can range from soft to hard and typically have high melting points, like sodium or titanium. Amorphous solids, including atoms, ions, molecules, or polymers like plastics, lack a distinct pattern and can flow, unlike typical solids, and do not have a clear melting point; glass and tar are notable examples. Understanding the properties of these solids is crucial for applications in various fields, from material science to engineering.

Crystalline vs Amorphous Solids

Solids can be fundamentally categorized into two main types: crystalline and amorphous solids. Crystalline solids are characterized by their highly ordered arrangements of atoms, ions, or molecules, while amorphous solids exhibit a random arrangement without any discernible pattern.

Crystalline solids can be further divided into four categories: ionic solids, molecular solids, covalent network solids, and metals. Ionic solids consist of ions held together by electrostatic forces, specifically the attraction between cations (positively charged ions) and anions (negatively charged ions). These solids are typically hard and brittle, with high melting points. A common example is sodium chloride (NaCl).

Molecular solids, on the other hand, have molecules as their smallest unit, and their structure is maintained by intermolecular forces. These solids tend to be softer and have lower to moderate melting points, with ice (H₂O) serving as a classic example.

Covalent network solids are composed of atoms connected by covalent bonds, making them some of the hardest materials known. They possess very high melting points, often exceeding those of ionic solids. Diamonds, a form of carbon, exemplify covalent network solids due to their exceptional hardness.

Metals consist of metal atoms bonded through metallic bonds, which involve the delocalization of electrons. Metals can vary in texture from soft, like sodium, to hard, like titanium, and generally have high melting points. They are also known for their lustrous appearance.

In contrast, amorphous solids lack a defined structure and can be made up of atoms, ions, molecules, or polymers, which include various types of plastics. Due to their irregular arrangement, amorphous solids do not have a distinct melting point and can exhibit flow, a property typically associated with liquids. Examples of amorphous solids include glass and tar, the latter being a sticky substance used in road construction.

Understanding these distinctions between crystalline and amorphous solids, along with their properties and examples, is essential for grasping the fundamental concepts of solid-state chemistry.

Ionic Solid Identification Example

Ionic solids are compounds formed from the electrostatic attraction between positive and negative ions. Typically, the positive ion is a metal or the ammonium ion, while the negative ion is a non-metal. In identifying an ionic solid, one should look for a combination of these ions.

For example, in the case of aluminum fluoride (AlF₃), aluminum acts as the positive ion, being a metal, and fluorine serves as the negative ion, being a non-metal. This combination exemplifies the characteristics of an ionic compound, making aluminum fluoride a suitable example of an ionic solid.

Thus, when evaluating options for ionic solids, the presence of a metal paired with non-metals is key to determining the correct answer.

What is the major electrostatic force found between ammonia molecules, NH₃?

• A. a bond
• B. Ionic bond
• C. Covalent bond
• D. Intermolecular Forces
• E. Anionic bond

Answer: (D)

As it cools off, olive oil slowly hardens and forms a solid over a range of temperatures. Which best describes it as a solid? 

• A. Ionic
• B. Covalent Network
• C. Metallic
• D. Amorphous
• E. Molecular Crystals

Answer: (E)

Compound A is hard, doesn't conduct electricity, and melts at 1400ºC. Compound A represents which of the following:

• A. Ionic solid
• B. Metallic solid
• C. Molecular solid
• D. Covalent Network solid

Answer: (D)

Classify each solid as amorphous, molecular, network covalent, alloy or ionic. 
a) Steel                                               ______________________
b) CO₂                                                ______________________
c) Graphite                                         ______________________
d) CaCO₃                                            ______________________
e) Bronze, an alloy of Cu and Sn      ______________________

• A. Steel - alloy; CO₂ - molecular; Graphite - network covalent; CaCO₃ - ionic; Bronze - alloy
• B. Steel - metallic; CO₂ - molecular; Graphite - network covalent; CaCO₃ - ionic; Bronze - alloy
• C. Steel - alloy; CO₂ - network covalent; Graphite -network covalent; CaCO₃ - ionic; Bronze - metallic
• D. Steel - metallic; CO₂ - network covalent; Graphite - amorphous; CaCO₃ - network covalent; Bronze - alloy

Answer: (A)

Here’s what students ask on this topic:

What forces hold molecular solids together?

Molecular solids are held together by intermolecular forces, which are attractions between molecules. These forces include:

1. Van der Waals Forces: These are the weakest interactions and arise from temporary dipoles in molecules. They include London dispersion forces, which are present in all molecular solids, and dipole-dipole interactions, which occur between polar molecules.
2. Hydrogen Bonds: A stronger type of dipole-dipole interaction, hydrogen bonds form when a hydrogen atom covalently bonded to a highly electronegative atom, like oxygen or nitrogen, is attracted to another electronegative atom in a different molecule.
3. Permanent Dipole Interactions: These occur when polar molecules with permanent dipoles attract each other. The positive end of one molecule is attracted to the negative end of another.

The relative strength of these forces determines the physical properties of the molecular solid, such as melting point, boiling point, and hardness. For example, ice, which is a molecular solid of water, has a relatively high melting point due to the strong hydrogen bonds between water molecules.

What structural units make up ionic solids?

Ionic solids are made up of a repeating pattern of ions bonded together by strong electrostatic forces known as ionic bonds. These structural units consist of positively charged ions, called cations, and negatively charged ions, called anions. The cations and anions are arranged in a regular, three-dimensional geometric lattice structure, which maximizes the attractive forces between oppositely charged ions while minimizing the repulsive forces between ions of the same charge.

The specific arrangement of ions within the lattice depends on the sizes and charges of the ions involved. For example, sodium chloride (NaCl) forms a cubic lattice where each sodium ion (Na⁺) is surrounded by six chloride ions (Cl^-), and vice versa. This highly organized structure contributes to the characteristic properties of ionic solids, such as high melting and boiling points, brittleness, and the ability to conduct electricity when melted or dissolved in water.

What are atomic solids?

Atomic solids are a class of solids composed of atoms held together by various types of bonding forces. Unlike molecular solids, where discrete molecules are held together by intermolecular forces, atomic solids consist of atoms linked in a continuous network. There are three main types of atomic solids, categorized by the type of bonding that holds the atoms together:

1. Metallic solids: In these solids, atoms are bonded by metallic bonds, which involve a "sea" of delocalized electrons that are shared among a lattice of metal cations. This bonding gives metallic solids their characteristic properties such as electrical conductivity, malleability, and ductility.
2. Network covalent solids: These solids, also known as covalent-network solids, have atoms connected by covalent bonds in a large, continuous network. Examples include diamonds (carbon atoms bonded to other carbon atoms) and quartz (silicon atoms bonded to oxygen atoms). These solids are typically very hard and have high melting points due to the strength of the covalent bonds.
3. Ionic solids: These are composed of positively and negatively charged ions held together by ionic bonds. The electrostatic attraction between the oppositely charged ions forms a crystal lattice structure. Ionic solids, like table salt (NaCl), tend to have high melting points and are brittle in nature.

Each type of atomic solid exhibits distinct physical properties.