Question

How do ionic solids differ in structure from molecular solids? What are the fundamental particles in each? Give two examples of each type of solid and indicate the individual particles that make up the solids in each of your examples.

Short answer

Ionic solids consist of charged ions arranged in a regular three-dimensional lattice and are held together by ionic bonds. In contrast, molecular solids are composed of discrete molecules held together by intermolecular forces (van der Waals, dipole-dipole, or hydrogen bonds). The fundamental particles in ionic solids are the charged ions, while in molecular solids, they are individual molecules. Examples of ionic solids include sodium chloride (NaCl) with sodium ions (Na+) and chloride ions (Cl-), and magnesium oxide (MgO) with magnesium ions (Mg2+) and oxide ions (O2-). Examples of molecular solids include water (H2O, ice) with water molecules (H-O-H) and carbon dioxide (CO2, dry ice) with carbon dioxide molecules (O=C=O).

Step-by-step solution

Define Ionic Solids and Molecular Solids

Ionic solids are composed of positively and negatively charged ions arranged in a regular three-dimensional lattice. The attraction between the oppositely charged particles is known as an ionic bond. In contrast, molecular solids consist of discrete molecules held together by one of the following intermolecular forces: van der Waals forces, dipole-dipole attractions, or hydrogen bonds.

Identify Fundamental Particles

In ionic solids, the fundamental particles are the positively and negatively charged ions. In molecular solids, the fundamental particles are the individual molecules, which are composed of atoms connected by covalent bonds.

Examples of Ionic Solids

Example 1: Sodium chloride (NaCl) In sodium chloride, the sodium ions (Na+) and chloride ions (Cl-) are the fundamental particles. These ions are arranged in a regular lattice structure, which gives the solid its characteristic properties, such as high melting and boiling points. Example 2: Magnesium oxide (MgO) In magnesium oxide, the magnesium ions (Mg2+) and oxide ions (O2-) are the fundamental particles. Similar to sodium chloride, these ions are arranged in a regular lattice structure, providing the solid with its high melting and boiling points.

Examples of Molecular Solids

Example 1: Water (H2O, solid form is known as ice) In ice, the fundamental particles are the individual water molecules, which consist of two hydrogen atoms bonded to one oxygen atom (H-O-H). These molecules are held together by hydrogen bonds, which give ice its crystalline structure and lower melting and boiling points when compared to ionic solids. Example 2: Carbon dioxide (CO2, solid form is known as dry ice) In dry ice, the fundamental particles are the individual carbon dioxide molecules, which consist of one carbon atom double-bonded to two oxygen atoms (O=C=O). The molecules are held together by van der Waals forces, making the solid's melting and boiling points even lower than ice.

Key concepts

Ionic Solids

Ionic solids are crystalline structures formed by the regular arrangement of ions which are held together by strong electrostatic forces known as ionic bonds. The fundamental particles in these solids are oppositely charged ions, such as the sodium cations (a^+) and chloride anions (Cl^-) in common table salt. Ionic solids often boast high melting and boiling points due to the considerable energy required to overcome the attraction between the ions.

To visualize this, imagine a rigid and organized network where each ion is tightly surrounded by ions of the opposite charge, resulting in the formation of a crystalline lattice structure. This arrangement allows ionic solids to maintain a stable form and contributes to their characteristic properties, such as brittleness, and the ability to conduct electricity when melted or dissolved in water.

Molecular Solids

Molecular solids are composed of discrete molecules held together by weaker intermolecular forces compared to the ionic bonds in ionic solids. The basic units of molecular solids are molecules, which are groups of atoms bonded covalently. The softness and relatively low melting and boiling points of molecular solids arise from these weaker intermolecular forces.

Consider water ice (H_2O), the solid form of water. Here, individual water molecules are linked by hydrogen bonds, establishing a less rigid structure than that found in ionic solids. Another example is dry ice, which consists of carbon dioxide (CO_2) molecules. These molecules interact through van der Waals forces, leading to an even more delicate lattice which easily breaks apart at relatively low temperatures.

Intermolecular Forces

Intermolecular forces are the attractions between molecules, influencing the physical properties of molecular solids. They include hydrogen bonds, dipole-dipole interactions, and van der Waals forces. Hydrogen bonds are strong attractions that occur when a hydrogen atom covalently bonded to a highly electronegative atom, like oxygen or nitrogen, is also weakly attracted to another electronegative atom in a neighboring molecule.

Dipole-dipole interactions occur between polar molecules, where partial positive charges are attracted to partial negative charges. Van der Waals forces, which include London dispersion forces, are the weakest intermolecular forces and exist between all molecules, even nonpolar ones. These forces are significant because they dictate molecular solids' melting and boiling points and their overall stability.

Crystalline Lattice Structure

The crystalline lattice structure refers to the highly organized, three-dimensional arrangement of particles in a solid. Both ionic and molecular solids form crystalline structures, but the nature of the particles and the forces holding them together differ significantly.

In ionic solids, the lattice is composed of alternating positive and negative ions, resulting in a structure that is efficient in maximizing attractive forces and minimizing repulsive forces. On the other hand, the lattice in molecular solids is formed by individual molecules and held together by intermolecular forces. Molecular solids' crystalline structures are generally less tightly bound than those of ionic solids, rendering them softer and less dense. The geometry of the lattice can greatly affect the physical properties of the solid, such as the angle of cleavage and the ability to refract light.