Ionic Compounds, Crystal Lattices, and Covalent Network Solids

Ionic Compounds and Lattice Energy

Formation and Properties of Ionic Compounds

Ionic compounds are formed from ions, which possess positive (cations) and negative (anions) charges. The electrostatic attraction between these oppositely charged ions results in the formation of ionic bonds. These bonds are typically strong and lead to high lattice energies and distinct physical properties.

• Ionic Bond: The force of attraction between oppositely charged ions in an ionic compound.

• Bond Dissociation Energy: The energy required to separate the ions in an ionic bond. Ionic bonds generally have higher bond dissociation energies compared to covalent bonds.

• Bond Length: Ionic bonds are usually shorter due to strong electrostatic forces.

Example Equations:

• Formation of NaCl: $\text{Na} + \frac{1}{2} \text{Cl}_2 \rightarrow \text{NaCl}$    $\Delta H = -411.12\ \text{kJ/mol}$

• Formation of CaO: $\text{Ca} + \frac{1}{2} \text{O}_2 \rightarrow \text{CaO}$    $\Delta H = -635.09\ \text{kJ/mol}$

Lattice Energy

Lattice energy is the energy released when ions come together to form a crystalline lattice. It is a measure of the strength of the ionic bonds in a solid. Lattice energy depends on the charges of the ions and the distance between them (ionic radii).

• Formula for Lattice Energy:

$\text{Lattice Energy} \propto \frac{Q_1 Q_2}{r}$

• $Q_1$ and $Q_2$ are the charges of the ions.

• $r$ is the distance between the centers of the ions (ionic radius).

• Higher charges and smaller ionic radii result in higher lattice energies.

Order of Lattice Energies (from notes):

• Al2O3 > NaCl, CaO, SO4

• NaCl < SrO < CaO < Al2O3

Additional info: The trend is based on both the magnitude of ionic charges and the size of the ions involved.

Crystal Lattices in Ionic Compounds

Structure and Arrangement

Ionic compounds do not have simple molecular geometries. Instead, they form crystal lattices, which are three-dimensional, repeating patterns of alternating positive and negative ions. This arrangement maximizes attractive forces and minimizes repulsive forces, resulting in a stable solid structure.

• Crystal Lattice: A regular, repeating arrangement of ions in a solid.

• Unit Cell: The smallest repeating unit in a crystal lattice.

• Common examples: NaCl (table salt), CaO, Al2O3, SO42- compounds.

Diagram: (Described) The notes include a ball-and-stick model and a space-filling model of a crystal lattice, showing alternating ions in a cubic arrangement.

Bond Dissociation and Lattice Energy Graphs

The bond dissociation energy of an ionic compound is often referred to as its lattice energy. Atomic radius also affects this energy: smaller ions lead to higher lattice energies.

• Graph: (Described) The notes show a graph of lattice energy versus atomic radius for halides, indicating that as atomic radius increases, lattice energy decreases.

Covalent Network Solids

Structure and Properties

Covalent network solids are substances in which atoms are connected by covalent bonds in a continuous network extending throughout the material. Unlike ionic compounds, these solids do not have discrete ions or charges, but are held together by strong covalent bonds in three dimensions.

• No +/- Charges: The atoms are neutral, and the bonding is covalent rather than ionic.

• Strong Bonds: Covalent bonds in three dimensions make these solids very hard and have high melting points.

• Examples: Diamond (carbon), Silicon dioxide (SiO2).

Diagram: (Described) The notes include a molecular model of diamond, showing carbon atoms connected in a tetrahedral network.

• Applications: Diamond is used in cutting tools and jewelry; SiO2 is found in quartz and glass.

Additional info: Covalent network solids are typically insulators or semiconductors due to the lack of free-moving electrons.