Question

Classify each of the following crystalline solids as ionic, molecular, or metallic: (a) sulfur, \(\mathrm{S}_{8}\) (b) sulfur dioxide, \(\mathrm{SO}_{2}\) (c) copper, Cu (d) silver nitrate, \(\mathrm{AgNO}_{3}\)

Short answer

(a) Molecular, (b) Molecular, (c) Metallic, (d) Ionic.

Step-by-step solution

Analyze Sulfur, \( \mathrm{S}_8 \)

The solid form of sulfur is composed of \( \mathrm{S}_8 \) molecules. These molecules are held together by van der Waals forces (a type of intermolecular force), which are characteristic of molecular solids. Thus, sulfur, \( \mathrm{S}_8 \), is a molecular solid.

Examine Sulfur Dioxide, \( \mathrm{SO}_2 \)

Sulfur dioxide is a molecular compound that forms molecular solids. In solid form, its molecules are held together by weak intermolecular forces rather than by ionic or metallic bonds, classifying \( \mathrm{SO}_2 \) as a molecular solid.

Assess Copper, Cu

Copper is composed of metal atoms arranged in a lattice structure where the electrons are delocalized, allowing them to move freely. This electron sea model is characteristic of metallic solids, so copper, Cu, is classified as a metallic solid.

Determine the Type of Silver Nitrate, \( \mathrm{AgNO}_3 \)

Silver nitrate is made up of silver ions \( \mathrm{Ag}^+ \) and nitrate ions \( \mathrm{NO}_3^- \). These ions are held together by ionic bonds within the solid state, making silver nitrate, \( \mathrm{AgNO}_3 \), an ionic solid.

Key concepts

Ionic Solids

Ionic solids are formed by the strong electrostatic forces of attraction between oppositely charged ions. These solids typically arise from the combination of metals and non-metals, where metals lose electrons to become positively charged ions (cations) and non-metals gain electrons to become negatively charged ions (anions). For example, in silver nitrate (\( \mathrm{AgNO}_3 \)), the silver ions \( (\mathrm{Ag}^+) \) and nitrate ions \( (\mathrm{NO}_3^-) \) are bonded together through these ionic bonds in a lattice structure.
The resulting structure is usually very stable and has a high melting point due to the strength of these bonds. Ionic solids also tend to be brittle because any applied force that shifts ions so that like charges meet will result in repulsion and the material breaking apart. Additionally, in solid form, ionic compounds do not conduct electricity. However, once melted or dissolved in water, their ions are free to move, and they can conduct an electrical current.

Molecular Solids

Molecular solids consist of molecules held together by intermolecular forces, which can include van der Waals forces, dipole-dipole interactions, and hydrogen bonds. These forces are much weaker than ionic or covalent bonds, leading to several distinct properties that molecular solids exhibit. Substances like sulfur \( (\mathrm{S}_8) \) and sulfur dioxide \( (\mathrm{SO}_2) \) fall into this category.
Molecular solids usually have lower melting and boiling points than ionic or metallic solids because the forces between molecules are comparatively weak. They are also generally soft and non-conductive due to the lack of free-moving charged particles. An interesting aspect of molecular solids is that their properties can vary significantly depending on the nature of the molecules and the specific intermolecular forces at play. This variability can be seen in how different types of molecular solids might be gases, liquids, or solids at room temperature.

Metallic Solids

Metallic solids are known for their unique structure, in which metal atoms are arranged in a highly organized lattice. This structure is defined by the 'electron sea model', where valence electrons are not bound to any particular atom, allowing them to move freely throughout the metal lattice. This model explains many of the characteristic properties of metals, such as their malleability, ductility, luster, and high thermal and electrical conductivity.
Copper \( (\mathrm{Cu}) \) is a prime example of a metallic solid. In metallic solids, the free electrons act as a 'glue' holding the positively charged metal ions together. This freedom of electron movement allows metals to conduct electricity efficiently as these electrons can easily move in response to an electric field. The delocalized nature of electrons also allows metals to be bent or shaped without breaking, as the metal ions are able to slide past each other while maintaining the metallic bond.