Question

List three kinds of crystalline solids, and give an example of each.

Short answer

Molecular: ice; Ionic: sodium chloride; Metallic: copper.

Step-by-step solution

Identify Molecular Solids

Molecular solids are crystals formed from molecules held together by intermolecular forces, such as Van der Waals forces or hydrogen bonds. These solids are usually soft with low melting points and poor electrical conductivity. An example of a molecular solid is ice (solid water).

Identify Ionic Solids

Ionic solids consist of ions held together by strong electrostatic forces (ionic bonds), forming a lattice structure. These solids tend to be hard and brittle with high melting points and usually conduct electricity when molten or dissolved in water. An example of an ionic solid is sodium chloride (table salt).

Identify Metallic Solids

Metallic solids are formed by metal atoms bonded by delocalized electrons, which allow them to be good conductors of electricity and heat. These solids have a range of hardness and melting points but are typically malleable. An example of a metallic solid is copper.

Key concepts

Molecular Solids

Molecular solids are a type of crystalline solid where the individual units of the crystal are molecules. These molecules are held together by relatively weak intermolecular forces, such as Van der Waals forces, dipole-dipole interactions, or hydrogen bonds.
These forces are significantly weaker than ionic or covalent bonds, resulting in certain unique characteristics:

• They typically have low melting and boiling points since less energy is required to break the forces holding the molecules together.
• Molecular solids are usually soft and often form lovely patterns in their crystal structures.
• They are generally poor conductors of electricity because their molecules do not have charged particles available for conduction.

Ice, or solid water, is an excellent example of a molecular solid, where water molecules are held together by hydrogen bonds, forming a solid when temperatures drop below 0°C.

Ionic Solids

Ionic solids are made up of ions held together in a 3D lattice structure by strong electrostatic forces known as ionic bonds. These bonds occur between positively charged cations and negatively charged anions.
As a result of these robust ionic bonds, ionic solids have some notable characteristics:

• They typically have high melting and boiling points because significant energy is needed to separate the ions.
• Ionic solids tend to be hard, yet brittle. When force is applied, like charges may align resulting in repulsion and causing the solid to crack.
• In solid form, they are generally non-conductive and insulators, but when melted or dissolved in water, they become excellent conductors of electricity due to the mobility of the ions.

A quintessential example of an ionic solid is sodium chloride, commonly recognized as table salt. It forms when sodium ions (Na⁺) and chloride ions (Cl^-) bond together in a repeating lattice.

Metallic Solids

Metallic solids are comprised of metal atoms arranged closely together in a unique structure called a metallic lattice. This lattice is characterized by a 'sea of electrons' where electrons are free to move throughout the structure. This electron mobility gives metallic solids a set of special properties:

• They are excellent conductors of electricity and heat due to the delocalized electrons that can carry energy quickly through the structure.
• Metallic solids exhibit malleability and ductility, allowing them to be shaped and stretched without breaking. The atoms can move past each other while maintaining the metallic bond.
• They have varying degrees of hardness and melting points. Some metals like iron are very hard, while others like lead are quite soft.

Copper, with its ideal balance of electrical conductivity and malleability, is a common example of a metallic solid widely used in electrical wires and components.