Question

Are the bonds in aluminium chloride ionic?

Answer 1

The Al-Cl bond in AlCl₃ is not ionic — it is polar covalent.

The electronegativity of Al is 1.5. The electronegativity of Cl is 3.0.

So ΔEN = 1.5. This corresponds to a bond that is 57 % covalent and 43 % ionic.

Molten AlCl₃ is a poor conductor of electricity, unlike NaCl.

In the gas phase, AlCl₃ is a trigonal planar molecule, analogous to BCl₃.

AlCl₃ reacts vigourously with water to form Al(OH₂)₆³⁺:

AlCl₃ + 6H₂O → Al(OH₂)₆³⁺ + 3 Cl⁻

The O atoms of the water are covalently bonded to the Al, but the Al(OH₂)₆³⁺ ion reacts as if it were Al³⁺.

Many chemists often write Al³⁺ for simplicity instead of the more correct Al(OH₂)₆³⁺, just as they write H⁺ instead of H₃O⁺.

Answer 2

No, you can't classify the bonds in aluminium chloride as being ionic bonds, although they do have significant ionic character.

The thing to look for when trying to decide the nature of a bond is the difference in electronegativity (EN) between the two atoms. In this case, aluminium's electronegativity is 1.61 and chlorine's electronegativity is 3.16.

Now, a good rule of thumb is that any bond formed between two atoms that have a difference in electronegativity values greater than 1.6 is ionic, or, better said, has dominant ionic character.

In this case, the difference in EN values will be

`DeltaEN = "3.16 - 1.61" = "1.55"`

This corresponds to a bond that is 45% ionic, which implies that it should be considered polar covalent.

This is explained by the difference in charge and ion size between aluminium and chlorine.

When compared with the chloride anion, or `Cl^(-)`, the aluminium cation, or `Al^(3+)`, is smaller and has a bigger charge, which means it can easily polarize the electron cloud of the chloride anion.

This distorsion will result in the overlapping of both electron clouds, which is exactly what happens when covalent bonds are formed.