Comparing FeCl3 and AlCl3: Understanding Covalent Character in Transition Metal Chlorides

The comparison between FeCl3 (iron(III) chloride) and AlCl3 (aluminum chloride) often involves examining their ionic vs. covalent character. Factors such as charge density, ionic radius, and polarizability play crucial roles in determining which compound exhibits stronger covalent characteristics.

Ionic vs. Covalent Character

The covalent character in ionic compounds arises from the polarization of anions by cations. The greater the charge density of the cation, the more it can polarize the anions, leading to an increase in covalent character.

Charge Density

Fe3 : Iron in FeCl3 has a 3 charge and a relatively small ionic radius, resulting in high charge density.

Al3 : Aluminum in AlCl3 also has a 3 charge but has a slightly larger ionic radius compared to Fe3 .

Polarizability

Chloride ions (Cl-): Chloride ions are relatively large and can be polarized by cations. The ability of the cation to polarize the anion increases with the charge density of the cation.

Comparison of FeCl3 and AlCl3

While both Fe3 and Al3 have the same charge, the Fe3 has a higher charge density due to its smaller size. This results in the Fe3 being better at polarizing the Cl- ions compared to the Al3 ion.

Based on these considerations, FeCl3 is generally considered to be more covalent than AlCl3, due to the higher charge density of the Fe3 ion, which leads to greater polarization of the chloride ions.

Electron Cloud Distortion and Covalent Bond Formation

In a more detailed understanding, the chloride ions in both compounds can be imagined to be spheres of negatively charged “mist,” representing the electron clouds around the central nucleus. Because they have one electron “too many,” they are wibbly-wobbly and easily distorted with less than one nuclear charge per electron.

When the metals each lose 3 electrons, the remaining electrons are drawn closer to the nucleus since each electron now has a greater “share” of attraction from the nuclear charge with more than one nuclear charge per electron.

Al3 is much smaller than Fe3 because it has fewer electron shells. This property is quantified by the “charge to radius ratio” (e/r). The higher the value, the greater the ability to distort or ‘polarize’ the electron “mist” or cloud.

Think of electrostatic demonstrations with Van der Graaf generators and lightning conductors: electrical charge is dissipated faster from a sharp point. This is why you approach a charged sphere with an earthed needle point and why lightning strikes a lightning conductor rather than the tower. Al3 is a much sharper point in space than Fe3 so it affects the electric field of the chloride ions much more.

Both distort the wibbly-wobbly negative “mist” around the Cl- ion but Al3 does it most. When this distortion occurs, the “mist” of electrons wraps around the positive ion, leading to a degree of covalent bond formation. The “degree of covalency” for AlCl3 is greater than for FeCl3 for the reasons described above.

I hope this helps. Try to visualize what is going on; it aids in understanding and understanding is power!

Understanding saves a great deal of effort compared to trying to remember unrelated facts.