Definition of Ionic Compounds

What are Ionic Compounds?

Ionic compounds are compounds consisting of ions.

Two-element compounds are usually ionic when one element is a metal and the other is a non-metal. Examples include:

Ionic compounds can be more complicated than the two-element ones listed above. Examples of polyatomic ionic compounds include:

Although they consist of positively and negatively charged ions, ionic compounds are electrically neutral, because the charges are always equal and opposite.

(The other major group of compounds is molecular compounds: these consist of molecules rather than ions.)

Bonding in Ionic Compounds

In its purest form, ionic bonding is not directional. It can be regarded as simple positive-negative Coulombic attraction between point charges. This is different from covalent bonding, in which electrons are shared by atoms, forming directional bonds.

However, absolutely pure ionic bonding does not exist. There is always at least a small degree of covalent bonding character in ionic compounds.

The Formulas and Structures of Ionic Compounds

The unit cell of fluorite, CaF₂. Million upon million of repeated translations of this unit cell in 3-dimensions produces a visible crystal of fluorite.

In a molecular compound, such as, for example, water, H₂O, or ethanol, C₂H₅OH, each unit of the substance, the molecule, consists of the number of atoms shown in the formula.

In an ionic compound, such as common salt, NaCl, or magnesia MgO, the formula tells us the correct ratio of elements present, but it does not specify the unit. Ionic compounds exist as giant crystal lattices.

Each crystal contains an unspecified number of ions: the numbers are enormous: even a 0.5 gram crystal of sodium chloride contains about 1 x 10²² ions.

The structures of crystals can be understood in terms of their repeating units - unit cells: these can be determined by X-ray diffraction measurements.

Copper(II) sulfate hydrate crystals CuSO₄·5H₂O.
Image by Crystal Titan.

Crystals of sodium chloride, NaCl, magnified.

Electric Conductivity

In general, ionic solids do not carry electric current, because the ions are in fixed positions in the crystal lattice. They do, however, conduct electricity when molten or when dissolved in solution, when the ions are released from the crystal lattice and become mobile.

Solutions of Ionic Compounds

Ionic compounds dissolve best in polar solvents. Examples of polar solvents are water, methanol, and formamide.

Ionic compounds are insoluble or have very low solubility in non-polar solvents. Examples of non-polar solvents are hydrocarbons, and chloroform.

Solvation
For an ionic compound to dissolve, the electrostatic bonds holding the ions in their rigid crystal lattice must be replaced by attractions between ions and solvent molecules. Each ion becomes surrounded by solvent molecules, as shown in the diagram.

Solvation of ionic compounds in polar solvents is driven by a decrease in free energy.

Na⁺ Solvation Shell

When NaCl dissolves in water, the polar water molecules form solvation shells around the dissolved ions. Here, water's oxygen atoms, each with a slight negative charge, solvate the Na⁺

Cl^- Solvation Shell

Likewise, the Cl^- is surrounded by water molecules with their slightly positive hydrogen atoms oriented to the Cl^-.

Acids and Bases

Bronsted Lowry acids/bases are substances that can donate/accept H⁺ ions.

Substances containing H⁺ ions are classed as acids.

H⁺ acceptors such as OH^- and O^2- are classed as bases.