To save a lengthy story short, aluminium chloride exhibits covalent personality because

The aluminium ion has actually high fee densityThe electron cloud of the chloride is distortable (JC term, polarizable)The mix of the 2 factors over leads to some degree of electron sharing between the hopeful aluminium ion and also the an adverse chloride ion and hence the covalent character.

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In general, in order because that an ionic compound to have actually covalent personality (oxymoronic, ns know.. But.. ) the positive charge that the cation should be so strong that it causes the electron cloud the the anion to it is in distorted (fancy JC term, polarized) such the electrons end up being drawn right into the room between the positive cation and also the an adverse anion. Hence, the electron are claimed to it is in “shared” and also the ionic link is claimed to exhibit part covalent character. (See diagrams below)


Scenario 1: large cation = lower charge density, not enough to distort the electron cloud the the anion significantly.

Scenario 2: together the charge thickness of the cation rises (represented by smaller cation), the attractive strength of the cation increases and also the electron cloud of the anion becomes progressively distorted.


Scenario 3: once the charge density of the cation is high (represented through a small cation), the attractive strength of the cation is solid enough to distort the electron cloud of the anion such that some electrons room “shared” between the 2 ions.

Let’s take it a deeper look at the 2 factors described briefly above:

“Strength” of cation (JC term, polarizing power of cation)Ease the distortion that electron cloud that anion (JC term, polarizability that anion)

“Strength” of cation

“Strength” that cation is pertained to its charge density, which have the right to be loosely defined as charge separated by volume the cation. The bigger the fee density, the more “concentrated” the optimistic charge (the cation becomes more “powerful” so to speak), the more likely that is able to entice electrons indigenous the anion right into the space between the cation and also the anion, thereby developing “electron sharing” and also hence covalent character.

Why go aluminium ion have high fee density?

Firstly, aluminium ions have actually high fee of +3. Also, as contrasted to salt or magnesium, the ionic size of aluminium is smaller (All three ions have electronic structure 2,8. However, Al has 13 proton in the nucleus vs 12 and also 11 for Mg and Na respectively. For this reason the electrons of Al would be pulled closer to the nucleus because of the greater number of protons in the nucleus causing a smaller ionic size). Since, aluminium ion has actually high charge, low volume it has actually high charge density.

Ease that distortion that electron cloud that anion

Ease of distortion that electron cloud of anion counts on

Size of anionCharge the anion

Factor 1 is intuitive. The bigger the dimension of the anion, the more away the valence covering electrons will certainly be from the optimistic nucleus, the lesser the quantity of electrostatic attraction in between the optimistic nucleus and the valence shell electrons, the less complicated it will certainly be because that the valence electrons to it is in pulled away by the positive cation leading to the electron cloud to be distorted.

Comparing the digital structures of Fluoride (2,8) and chloride (2,8,8) you will realize that chloride has one extra electron shell, hence, the valence electron of chloride will be additional away from the positive nucleus as compared to fluoride and also it becomes easier for the valence electrons to it is in attracted far by a positive cation.

For element 2, a higher negative fee on the anion suggests that that is easier for the electron cloud to it is in distorted. We describe this using the instance of chloride and also sulphide ions (electronic framework of both: 2,8,8). Chlorine has actually 17 protons while sulphur only has 16. Therefore as compared to chlorine, sulphur will exert much less electrostatic attraction top top its valence electrons, making it less complicated for castle to be pulled away by a hopeful cation.

Putting the factors together:

Aluminium chloride is covalent because the aluminium ion has high charge density while the chloride ion is fairly polarizable. Prolonging this concept, you should discover that aluminium bromide, aluminium iodide would certainly all exhibit covalent character. Furthermore, due to the fact that the number of electron shells is increasing, the level of covalent character would increase as well (because that is even much easier for the electron cloud to be distorted. See variable 1. This leader to a greater level of “electron sharing” and hence a greater degree of covalent character).

Aluminium fluoride at the same time would stay ionic. This is because there is much less shielding effect as contrasted to the chloride ion. Hence, the electrostatic attraction in between the nucleus and also the valence electron is greater, making the more challenging for the electron cloud to be distorted/”shared”.

Also, you have to expect that various other cations with high fee such as Fe3+ will type compounds v covalent character. Therefore, compounds such together FeCl3, FeBr3 and FeI3 are all covalent as well.

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Some caveats:

Not much is known around aluminium astatide because astatine is a very unstable element. However if it is formed, you must expect the it has covalent character.

Theory predicts the aluminium sulphide (Al2S3) need to be covalent in nature (See factor 2). But, it remains ionic. However, the reason and also explanation because that this is means beyond what can reasonably be experiment on in the ‘O’ Levels. Hence, I will certainly not discuss it here. If you room really interested or girlfriend just have too much totally free time, I indicate you visit Mr. David’s blog!