Properties of ionic compounds Knowledge-Based Study

Study notes

1. The Ionic Lattice

   Ionic compounds consist of positively charged ions (cations) and negatively charged ions (anions) held together by strong, non-directional electrostatic forces of attraction known as ionic bonds.

   Rather than forming discrete, individual molecules, these ions assemble into a highly organized, repeating three-dimensional framework called a crystal lattice. In this regular array, each ion is surrounded by a fixed number of oppositely charged ions, maximizing attractive interactions while minimizing repulsions between like charges.

   High Thermal Stability

   Ionic compounds typically exhibit high melting points and high boiling points. This property directly reflects the strength of the electrostatic forces within the lattice:

   • To melt an ionic solid, a large amount of thermal energy is required to overcome the strong electrostatic attractions operating throughout the entire giant ionic network.
   • The magnitude of lattice energy, and consequently the melting point, depends primarily on the charges of the ions and their ionic radii. Compounds with smaller, higher-charged ions (such as $\text{MgO}$) have substantially higher melting points than compounds with larger, singly charged ions (such as $\text{NaCl}$).

   Mechanical Rigidity and Brittleness

   Although ionic crystals are hard and rigid, they are characteristically brittle rather than malleable or ductile:

   • When an external mechanical force is applied to an ionic crystal, layers of ions shift relative to one another.
   • This displacement forces ions of like charges into direct alignment with each other (cations next to cations, anions next to anions).
   • The resulting strong electrostatic repulsion causes the crystal to fracture cleanly along cleavage planes rather than deform.

   Conductivity in Different States

   Electrical conductivity requires the presence of mobile charge carriers, such as free electrons or mobile ions:

   • Solid state: Ionic compounds do not conduct electricity. The ions occupy fixed positions within the crystal lattice and are only capable of vibrating about these points, meaning no free mobile charges can carry a current.
   • Molten (liquid) state: When heated above their melting points, the lattice breaks down. The ions become freely mobile and can migrate toward oppositely charged electrodes, allowing the substance to conduct electricity.
   • Aqueous solution: When dissolved in water, the crystal lattice dissociates into mobile hydrated cations and anions, enabling the solution to function as an electrolyte.

   Solubility Trends

   Many common ionic compounds dissolve readily in polar solvents such as water, but are insoluble in nonpolar solvents such as hexane or benzene:

   • In water: The polar water molecules interact favorably with the surface ions through ion-dipole attractions. If the hydration energy released when water molecules surround the ions is sufficient to overcome the lattice energy of the crystal, dissolution occurs.
   • In nonpolar solvents: Nonpolar molecules interact with ions only via weak dispersion forces. These interactions are far too weak to overcome the substantial lattice energy, leaving the solid intact.