Aluminum chloride (\(\text{AlCl}_3\)) is a compound whose chemical bonding classification challenges the simple division between purely ionic and purely covalent substances. When chemists attempt to categorize a bond, they typically look for a clear electron transfer (ionic) or a clear electron sharing (covalent). The bonding in aluminum chloride is a classic example of a compound that exhibits characteristics of both types, making its nature complex.
This dual behavior demonstrates that many chemical bonds exist on a spectrum, with \(\text{AlCl}_3\) sitting squarely in the middle. Understanding this nuanced bonding requires looking beyond simple rules to the specific atomic interactions that govern the compound’s structure and properties.
Why It Looks Ionic on Paper
A basic way to predict bond type involves calculating the difference in electronegativity (END) between the two atoms involved. Electronegativity is a measure of an atom’s ability to attract a shared pair of electrons toward itself.
Aluminum (Al) has a Pauling electronegativity value of approximately 1.61, while Chlorine (Cl) has a value of about 3.16, resulting in a difference of 1.55. In introductory chemistry, a difference greater than 1.7 is often cited as the threshold for a bond to be considered predominantly ionic. Since the \(\text{Al-Cl}\) difference is close to this threshold, and because it is formed from a metal and a non-metal, this simple model initially suggests \(\text{AlCl}_3\) is an ionic compound.
The Shift to Covalent Character
The simple electronegativity difference fails to account for powerful electronic interactions that shift the bond toward covalent character. This phenomenon is explained by Fajan’s Rules, which describe how an ionic bond acquires covalent properties through polarization. Polarization is the distortion of the anion’s electron cloud by the cation.
The aluminum cation (\(\text{Al}^{3+}\)) is small and carries a high \(+3\) charge, giving it high polarizing power. This concentrated charge creates a strong electric field around the ion. Conversely, the chloride ion (\(\text{Cl}^{-}\)) is large and highly polarizable because its outer electrons are loosely held.
The highly charged \(\text{Al}^{3+}\) ion strongly pulls on the \(\text{Cl}^{-}\) electron cloud, causing it to distort and overlap with the aluminum ion’s space. This distortion results in significant electron sharing, which is the definition of a covalent bond. Consequently, the \(\text{Al-Cl}\) bond is better described as highly polar covalent.
Structural Evidence: From Solid to Gas
The physical structure of aluminum chloride provides tangible evidence of its mixed bonding nature, which changes dramatically with its physical state. In its solid, anhydrous form, \(\text{AlCl}_3\) exists as a sheet-like, layered lattice. Within this lattice, each aluminum atom is surrounded by six chlorine atoms, forming an octahedral coordination geometry.
This extended, polymeric arrangement is characteristic of an ionic compound, where a network of ions is held together by strong electrostatic forces. However, when heated, the structure breaks down, confirming weaker, more covalent interactions. Solid aluminum chloride sublimes around \(180^\circ \text{C}\), bypassing the liquid state under normal atmospheric pressure.
The gaseous phase, and the liquid phase under pressure, exist predominantly as a molecule with the formula \(\text{Al}_2\text{Cl}_6\), known as a dimer. This dimer consists of two \(\text{AlCl}_3\) units joined by two bridging chlorine atoms. The formation of a discrete, small molecule like a dimer, rather than an extended ionic lattice, is a hallmark of covalent bonding. The bonds within the \(\text{Al}_2\text{Cl}_6\) molecule are demonstrably covalent, including coordinate covalent bonds where the bridging chlorine atoms donate electron pairs to the electron-deficient aluminum atoms.
Physical Properties of Aluminum Chloride
The macroscopic behavior of aluminum chloride reflects its underlying covalent characteristics, contrasting sharply with truly ionic compounds. Classic ionic compounds, like sodium chloride (\(\text{NaCl}\)), have very high melting points.
In contrast, \(\text{AlCl}_3\) sublimes at a remarkably low temperature (approximately \(178^\circ \text{C}\) to \(192^\circ \text{C}\)). This low sublimation point is typical of molecular compounds, which require little energy to break the relatively weak intermolecular forces holding the discrete molecules together. Furthermore, while melted ionic salts conduct electricity efficiently due to free ions, molten aluminum chloride is a poor conductor. The compound also exhibits solubility in non-polar organic solvents, another trait associated with covalent molecules.