Question

On the basis of the smaller units that make up the crystals, cite three types of crystalline solids. For each type of crystalline solid, give an example of a substance that forms that type of solid.

Short answer

Three types of crystalline solids are ionic solids, covalent (network) solids, and molecular solids. An example of an ionic solid is sodium chloride (NaCl); it has strong electrostatic forces between ions. A covalent (network) solid example is diamond, which has carbon atoms held together by covalent bonds in a three-dimensional network. Lastly, a molecular solid example is ice (H₂O), where water molecules are held together by hydrogen bonds.

Step-by-step solution

Identify the three types of crystalline solids

There are four types of crystalline solids based on the types of constituent particles and the forces holding them together: ionic solids, covalent (network) solids, molecular solids, and metallic solids. Since the exercise asks for three types, we will discuss ionic, covalent, and molecular solids in detail.

Describe the properties of ionic solids

Ionic solids are crystalline solids composed of positively charged cations and negatively charged anions that are held together by strong electrostatic forces (ionic bonds). These solids have high melting and boiling points due to the strong ionic bonds between the ions. They are generally brittle and poor conductors of electricity when in solid form, but they can conduct electricity in molten state or when dissolved in water.

Provide an example of an ionic solid

A common example of an ionic solid is sodium chloride (NaCl), also known as table salt. In its crystalline structure, each sodium ion (Na⁺) is surrounded by six chloride ions (Cl⁻), and each chloride ion is surrounded by six sodium ions, forming a repeating cubic lattice pattern.

Describe the properties of covalent (network) solids

Covalent (network) solids are crystalline solids in which atoms are held together by a network of covalent bonds. These solids tend to have high melting and boiling points due to the strong covalent bonds holding the atoms together. They are typically hard and brittle, and they are poor conductors of electricity.

Provide an example of a covalent (network) solid

A well-known example of a covalent (network) solid is diamond, which is made of carbon atoms. In diamond, each carbon atom is covalently bonded to four other carbon atoms, forming a strong three-dimensional network.

Describe the properties of molecular solids

Molecular solids are crystalline solids that are made up of discrete molecules held together by intermolecular forces such as hydrogen bonding, dipole-dipole interactions, and dispersion (van der Waals) forces. They generally have low melting and boiling points as these intermolecular forces are weaker than ionic or covalent bonds. Molecular solids are typically soft, and they are poor conductors of electricity.

Provide an example of a molecular solid

One example of a molecular solid is ice, the solid form of water (H₂O). In ice, water molecules are held together by hydrogen bonds, which are responsible for the unique properties of this molecular solid.

Key concepts

Ionic Solids

Ionic solids are fascinating crystalline structures composed of charged ions. These ions are either positively charged cations or negatively charged anions. The attraction between these opposite charges results in strong electrostatic forces, which hold the ions in place. This force is known as an ionic bond.
Ionic solids have distinct properties:

• High melting and boiling points due to the strength of ionic bonds.
• Hard and brittle nature, which means they can crack or shatter rather than deform when force is applied.
• Poor electrical conductors in their solid state, but they can conduct electricity when melted or dissolved in water, as the ions are free to move.

A classic example of an ionic solid is sodium chloride (NaCl), commonly known as table salt. Its structure is a lattice where sodium ions (Na⁺) and chloride ions (Cl⁻) alternate in a repeating pattern, forming a rigid cubic network.

Covalent Network Solids

Covalent network solids are a unique type of crystalline solid where atoms are connected in a continuous network by covalent bonds. The strength and directionality of these bonds give covalent network solids their specific characteristics.
These solids exhibit notable properties:

• Very high melting and boiling points because breaking the covalent network requires significant energy.
• Typically hard and brittle, resistant to scratching and wear.
• Poor electrical conductors because there are no free-moving charged particles within the solid.

Diamond is a famous example of a covalent network solid. In diamond, each carbon atom is bonded tetrahedrally to four other carbon atoms, resulting in an exceptionally hard structure. This arrangement contributes to diamond's renowned hardness and its use in cutting tools.

Molecular Solids

Molecular solids are composed of molecules held together by relatively weak intermolecular forces. These forces include hydrogen bonds, dipole-dipole interactions, and London dispersion forces.
Molecular solids have characteristics that distinguish them from other types of crystalline solids:

• Lower melting and boiling points compared to ionic and covalent network solids. This is due to weaker forces between molecules.
• Softer nature, often leading to ease of deformation under pressure.
• Poor conductors of electricity because there are no ions or delocalized electrons to carry charge.

A well-known example of a molecular solid is ice. In its solid state, water molecules are arranged in a lattice, held together by hydrogen bonds which give ice its unique properties, such as being less dense than its liquid form.