Ionic compounds form when a metal atom transfers one or more electrons to a nonmetal atom, creating charged particles called ions. The atom that loses electrons becomes a positively charged cation, while the atom that gains them becomes a negatively charged anion. The resulting chemical linkage, known as an ionic bond, is the powerful electrostatic attraction between these oppositely charged ions. The strength of this force dictates the physical and chemical behaviors that characterize ionic compounds.
Atomic Structure and Lattice Formation
The strong electrostatic attraction between cations and anions prevents the formation of individual, neutral molecules. Instead, the ions arrange themselves into a vast, repeating, three-dimensional geometric structure called a crystal lattice. This highly ordered structure maximizes attractive forces between opposite charges and minimizes repulsion between like charges. The specific geometry of the lattice depends on the relative sizes and charges of the constituent ions.
Because the structure is an extended network, the chemical formula represents a formula unit rather than a collection of separate units. The formula unit indicates the simplest whole-number ratio of ions required to achieve overall electrical neutrality. For example, in magnesium chloride, the formula MgCl2 shows that two chloride anions are required for every one magnesium cation to balance the charges.
High Melting Points and Mechanical Strength
The crystal lattice’s robust nature results in the high thermal stability of ionic compounds. A substantial amount of energy must be supplied to overcome the strong electrostatic forces holding the ions rigidly in their fixed positions. This requirement translates to very high melting and boiling points. For instance, sodium chloride melts at approximately \(801^\circ\text{C}\), demonstrating the energy needed to transition the substance from a solid to a liquid.
Ionic solids are typically hard but exhibit a characteristic brittleness. The hardness is a consequence of the strong attractive forces that resist deformation. However, when stress is applied, it can cause one layer of ions to shift slightly relative to an adjacent layer. This displacement brings like-charged ions into alignment, creating an intense, localized electrostatic repulsion that overcomes the attractive forces. This sudden repulsive force causes the crystal to fracture or cleave cleanly along specific planes.
Electrical Conductivity and Solubility
The ability of an ionic compound to conduct electricity depends entirely on its physical state. In the solid form, the charged ions are locked into fixed positions within the lattice and cannot move freely. Since mobile charged particles are required to carry an electric current, solid ionic compounds act as electrical insulators.
Conductivity changes dramatically when the solid lattice is broken down. When an ionic compound is melted or dissolved, the ions become completely mobile. In the molten state, heat overcomes the lattice forces, freeing the ions to move. When dissolved in water, the ions dissociate and are free to migrate toward an oppositely charged electrode, allowing the substance to conduct electricity efficiently.
Ionic compounds show high solubility in polar solvents, such as water, but low solubility in nonpolar solvents. Water is a polar molecule, meaning it has slightly positive and negative ends. When an ionic solid is introduced to water, the polar molecules surround the individual ions, pulling them away from the crystal lattice. This process, called solvation or hydration, effectively shields the ions and allows the compound to dissolve. Nonpolar solvents cannot exert the necessary attractive forces to disrupt the strong electrostatic bonds of the lattice, leaving the ionic compound insoluble.