To keep a long story short, aluminium chloride exhibits covalent character becauseThe aluminium ion has high charge densityThe electron cloud of the chloride is distortable (JC term, polarizable)The combination of the 2 factors above leads to some degree of electron sharing between the positive aluminium ion and the negative chloride ion and hence the covalent character.
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In general, in order for an ionic compound to have covalent character (oxymoronic, I know.. but.. ) the positive charge of the cation must be so strong that it causes the electron cloud of the anion to be distorted (fancy JC term, polarized) such that electrons become drawn into the space between the positive cation and the negative anion. Hence, the electrons are said to be “shared” and the ionic compound is said to exhibit some covalent character. (See diagrams below)
Scenario 1: Large cation = lower charge density, not enough to distort the electron cloud of the anion significantly.
Scenario 3: When the charge density of the cation is high (represented by a small cation), the attractive power of the cation is strong enough to distort the electron cloud of the anion such that some electrons are “shared” between the 2 ions.
Let’s take a deeper look at the 2 factors described briefly above:“Strength” of cation (JC term, polarizing power of cation)Ease of distortion of electron cloud of anion (JC term, polarizability of anion)
“Strength” of cation
“Strength” of cation is related to its charge density, which can be loosely defined as charge divided by volume of cation. The larger the charge density, the more “concentrated” the positive charge (the cation becomes more “powerful” so to speak), the more likely it is able to attract electrons from the anion into the space between the cation and the anion, thereby creating “electron sharing” and hence covalent character.
Why does aluminium ion have high charge density?
Firstly, aluminium ions have high charge of +3. Also, as compared to sodium or magnesium, the ionic size of aluminium is smaller (All three ions have electronic structure 2,8. However, Al has 13 protons in the nucleus vs 12 and 11 for Mg and Na respectively. Hence the electrons of Al would be pulled closer to the nucleus due to the greater number of protons in the nucleus resulting in a smaller ionic size). Since, aluminium ion has high charge, low volume it has high charge density.
Ease of distortion of electron cloud of anion
Ease of distortion of electron cloud of anion depends onSize of anionCharge of anion
Factor 1 is intuitive. The larger the size of the anion, the further away the valence shell electrons will be from the positive nucleus, the lesser the amount of electrostatic attraction between the positive nucleus and the valence shell electrons, the easier it will be for the valence electrons to be pulled away by the positive cation causing the electron cloud to be distorted.
Comparing the electronic structures of Fluoride (2,8) and chloride (2,8,8) you will realize that chloride has one extra electron shell, hence, the valence electrons of chloride will be further away from the positive nucleus as compared to fluoride and it becomes easier for the valence electrons to be attracted away by a positive cation.
For factor 2, a higher negative charge on the anion implies that it is easier for the electron cloud to be distorted. We explain this using the example of chloride and sulphide ions (electronic structure of both: 2,8,8). Chlorine has 17 protons while sulphur only has 16. Thus as compared to chlorine, sulphur will exert less electrostatic attraction on its valence electrons, making it easier for them to be pulled away by a positive cation.
Putting the factors together:
Aluminium chloride is covalent because the aluminium ion has high charge density while the chloride ion is relatively polarizable. Extending this concept, you should find that aluminium bromide, aluminium iodide would all exhibit covalent character. Furthermore, because the number of electron shells is increasing, the degree of covalent character would increase as well (because it is even easier for the electron cloud to be distorted. See factor 1. This leads to a greater degree of “electron sharing” and hence a greater degree of covalent character).
Aluminium fluoride on the other hand would remain ionic. This is because there is much less shielding effect as compared to the chloride ion. Hence, the electrostatic attraction between the nucleus and the valence electrons is greater, making it more difficult for the electron cloud to be distorted/”shared”.
Also, you should expect that other cations with high charge such as Fe3+ will form compounds with covalent character. Therefore, compounds such as FeCl3, FeBr3 and FeI3 are all covalent as well.
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Not much is known about aluminium astatide because astatine is a very unstable element. But if it is formed, you should expect that it has covalent character.
Theory predicts that aluminium sulphide (Al2S3) should be covalent in nature (See factor 2). But, it remains ionic. However, the reason and explanation for this is way beyond what can reasonably be tested on in the ‘O’ Levels. Hence, I will not discuss it here. If you are really interested or you just have too much free time, I suggest you visit Mr. David’s blog!