When examining the chemical formula \(\text{AlCl}_3\), or aluminum chloride, the question of whether its bonds are ionic or covalent reveals a fundamental complexity in chemistry. Chemical bonds rarely fall neatly into one category, existing instead on a spectrum between the two ideal extremes of bonding. Aluminum chloride is a classic example of a compound that defies simple classification, exhibiting characteristics of both ionic and covalent compounds depending on the circumstances. Understanding the behavior of \(\text{AlCl}_3\) requires moving beyond basic definitions to appreciate the subtle interplay of electron attraction and repulsion within the compound.
Defining Ionic and Covalent Bonds
Chemical bonds are generally categorized based on how electrons are distributed between the participating atoms. An ionic bond involves the complete transfer of valence electrons from one atom to another, typically occurring between a metal and a non-metal. This transfer creates oppositely charged ions, which are held together by strong electrostatic attraction.
In contrast, a covalent bond involves the mutual sharing of valence electrons between two atoms, usually two non-metals. The primary tool chemists use to predict the nature of a bond is the difference in electronegativity between the two atoms. Electronegativity is an atom’s ability to attract electrons toward itself within a bond. A large difference, generally above 1.7 on the Pauling scale, suggests a bond is predominantly ionic, while a smaller difference indicates electrons are shared, resulting in a covalent bond.
The Borderline Case of Aluminum Chloride
Applying the electronegativity rule to aluminum chloride initially suggests an ionic compound, as aluminum is a metal and chlorine is a non-metal. The difference in electronegativity between aluminum and chlorine is about 1.55, which technically falls into the range for a polar covalent bond, though it is close to the ionic threshold. Despite this calculation, the physical and chemical behavior of anhydrous \(\text{AlCl}_3\) suggests a strong covalent character.
This covalent nature is explained by the concept of polarization. When aluminum forms a theoretical \(\text{Al}^{3+}\) ion, it is very small and carries a high positive charge. This small, highly charged cation intensely attracts and distort the electron cloud of the larger chloride anion (\(\text{Cl}^{-}\)). Instead of a clean transfer, the chloride electron cloud is pulled back toward the aluminum nucleus, resulting in significant electron sharing.
The resulting distortion is so substantial that the bond acquires a strong covalent character. This phenomenon is pronounced with small cations carrying a high charge, like aluminum, and larger, easily deformable anions, like chloride. Therefore, the bond in \(\text{AlCl}_3\) is highly polar and is best described as a polar covalent bond, possessing about 55% covalent character and 45% ionic character.
Bonding Behavior Across Physical States
The complex bonding character of aluminum chloride is physically demonstrated by how its structure changes with temperature and state. In the solid state, \(\text{AlCl}_3\) forms a layered crystal structure where each aluminum atom is surrounded by six chlorine atoms, resembling an ionic lattice arrangement. However, the solid compound does not conduct electricity because the electrons are localized and the ions are not free to move.
A primary feature of \(\text{AlCl}_3\) is its remarkably low melting and boiling point compared to true ionic salts like sodium chloride. At normal atmospheric pressure, aluminum chloride bypasses the liquid state entirely, subliming directly from a solid to a gas at about \(180^\circ\text{C}\). This low sublimation point is characteristic of a molecular substance, not a high-melting ionic solid.
Upon heating, the solid lattice breaks down, and the compound transitions into distinct molecules. Specifically, it forms a double-molecule structure known as a dimer, represented by the formula \(\text{Al}_2\text{Cl}_6\). In this dimer, the aluminum atoms achieve a more stable tetrahedral coordination geometry, held together by shared, covalent bonds. This structural change confirms the dominant character of aluminum chloride is molecular and covalent.