Ionic Compounds: Salt Bridge, Coordination Number, Properties

Salt Bridge and Ionic Bonding

Ions are atoms or groups of atoms that have positive or negative charges due to the loss or gain of electrons. According to their electronic structure, each atom gives up or receives a certain number of electrons to achieve the stable noble gas configuration. They acquire a positive or negative charge; this is called the ionic valence. The ionic valence of an element is the charge acquired by its atoms when they become positive or negative ions. Atoms become positive or negative ions by transfer of electrons. Consequently, electrostatic forces cause ions to attract ions of opposite charge. The result of these forces is called a salt bridge.

The salt bridge is the union resulting from electrostatic forces between positive and negative ions, leading to the formation of a compound with a crystal lattice — an ionic solid — which is very stable. Many properties, such as solubility in water or electrical conductivity in aqueous solution or when melted, cannot be justified unless one accepts that ions (positive and negative) are formed instead of neutral atoms or molecules. These are ionic compounds.

Coordination Number in Ionic Lattices

Different ionic compounds adopt different crystal structures in which the characteristic ions are placed in specific positions (nodes) of the lattice, balancing attractive and repulsive forces. This structure must meet two conditions:

• Maximum packing: ions should occupy the smallest possible volume.
• Electrical neutrality: the total positive charge must equal the total negative charge.

The coordination number of an ion in an ionic lattice is the number of oppositely charged ions surrounding it at the same distance.

Properties of Ionic Compounds

Key properties include:

Thermal Stability

Thermal stability: Ionic compounds are typically solids at room temperature and have high melting and boiling points due to the strong attractive forces between ions.

Solubility and Solvation

Solubility: Many ionic compounds are soluble in water or other polar solvents because solvent molecules are polar. When an ionic solid is introduced into water, the ionic network breaks due to interactions between the ions in the crystal and the solvent dipoles. Ions enter the solution surrounded by H2O molecules and become solvated.

Energetically, two factors must be considered:

• Breaking the ionic network requires energy equal to the absolute value of the lattice energy, U.
• Solvation of ions releases (or requires) energy equal to the enthalpy (heat) of solvation.

Electrical Conductivity and Electrolysis

Electrical conductivity: Ionic compounds are not electrically conductive in the solid state because the ions are tightly bound in the crystal lattice and cannot move to carry charge. This changes when these compounds are melted or dissolved: the ions can move under an applied electric field and migrate to the electrodes. The process is called electrolysis, and such compounds in solution or molten state are called electrolytes.