AlI3 Lewis Structure & Characteristics: 15 Complete Facts

AlI₃ or triiodide aluminum is the halogenated molecule of Al having a molar mass of 407.695 g/mol for anhydrate form. Let us learn about AlI3 in detail.

Aluminum triiodide or AlI₃ is the monomeric form, it exists as a dimeric form because there is electron deficiency present over the Al atom. the molecular formula of the dimeric form is Al₂I₆, and the nature of bonding in the dimeric form is a 3c-4e model. It also exists in the hydrated form AlI₃.6H₂O.

The lattice structure of AlI₃ is monoclinic.  In this article, we can explore more about AlI₃ bonding mode, structure, polarity, and other important facts with proper explanation.

1.      How to draw AlI₃ lewis structure

AlI₃ lewis structure can help us to find out the valence electrons, structure, and other molecular properties. Let’s try to draw the AlI₃ lewis structure in a few steps.

Counting the valence electrons

At first, we should count the total valence electrons for AlI₃ which are 24. Those are the total number of valence electrons for the Al and three Iodine atoms. So, the total number of valence electrons is the summation of the valence electrons for the constituent atoms.

Choosing the central atom

Choosing central atoms is the one of most important steps for lewis structure drawing. Al is decided as the central atom because the size of the Al is larger than Iodine and also it is more electropositive than Iodine. All three iodine lie in the surrounding of the central Al.

Satisfying the octet

Each atom should be satisfied by its octet in a molecule. So, Al and iodine also try to complete their octet by fulfilling their valence shell with several eight electrons as they are p-block elements. So, the total number of electrons required is 4*8 = 32.  But the valence electrons are available 24.

Satisfying the valency

To complete the octet each atom should be satisfied by its valency first. The stable valency of Al is 3 and for Iodine is 1. So, the remaining 32-24 = 8 electrons should be distributed between 8/2 = 4 bonds. But the maximum possible bonds can occur at 3, so Al is not complete its octet here.

Assign the lone pairs

The remaining valence electrons which are non-bonded exist as lone pairs over the respective atoms. Iodine has seven valence electrons but only one electron is being used for bond formation with Al, so the remaining six electrons exist as three pairs of lone pairs. Al lacks lone pair.

2.      AlI₃ valence electrons

The valence electrons are the electrons present in the outermost orbital by the electronic configuration. Let’s count the total valence electrons for the AlI₃.

The total valence electrons for AlI₃ are 24 which are summation of the valence electrons of Al and three iodines. Al has three electrons in the valence shell and iodine has seven. So, total valence electrons for a molecule are always a summation of the valence electrons of the constituent atoms.

• Let us calculate the total valence electrons for the AlI₃ molecule,
• The valence electrons for the Al are 3
• The valence electrons for each iodine are 7
• So, in the AlI₃ the total valence electrons are 7*3 + 3 = 24 (three iodines are present).

3.      AlI₃ lewis structure lone pairs

Lone pairs are the available electrons after bond formation in the valence orbital or outermost shell. Let’s find out the lone pairs of the AlI₃.

The total lone pairs over the AlI₃ molecule are 18, which are present over only those atoms that have more electrons in the valence shell after bond formation. So when, valence electrons are greater than bonded electrons only then lone pairs are available. In this case, only Iodine contains lone pairs.

• Now count the lone pairs over the AlI₃ molecule by the formula, non-bonded electrons = valence electrons – bonded electrons.
• Lone pairs over Al atoms are, 3-3 = 0
• Lone pairs over each Iodine atom are, 7-1 = 6
• So, each iodine contains six lone pairs.
• The total lone pairs over the AlI₃ molecule are 0+ 6*3 = 18 lone pairs.

4.      AlI₃ lewis structure octet rule

Satisfying the valency after bond formation and completion of the outermost orbital by suitable electrons known as an octet. Let’s see how AlI₃ completes its octet.

To complete octet Al and iodine need eight electrons in their valence shell because they both belong to the p block. From their electronic configuration [Ne]3s²3p¹ and [Kr]4d¹⁰5s²5p⁵ it is evident that Al and Iodine need 5 and 1 electrons respectively. But the available electrons which can be shared will be 8.

Among those 8 electrons 5 for Al and 1 for each iodine. But Al can make maximum here bonds via a share of six electrons. So, there is a lack of two electrons for Al, and for this reason, AlI₃ exists as a dimer where two Al shared two electrons and complete the octet for both Al centers. I fulfill its octet.

5.      AlI₃ lewis structure shape

Lewis structure shape is the molecular shape for a particular molecule and what shape it adopts after arranging the atoms. Let’s discuss the AlI₃ shape in detail.

The monomeric structure of the AlI₃ is trigonal planar which is confirmed by the following table.

From the VSEPR (Valence Shell Electrons Pair Repulsion) theory, it is confirmed that AlI₃ is categorized as an AX₃ molecule. For that type of molecule, the shape and the structure are trigonal planar, so AlI₃ adopts ideal geometry and no deviation occurred.

6.      AlI₃ lewis structure angle

Bond angle or lewis structure angle is that angle that makes by the atom via the proper orientation of rearrangement. Let’s calculate the bond angle of AlI₃.

The bond angle I-Al-I is 120⁰ and there is no deviation occurred here. From the VSEPR it is clear that the shape of the molecule is trigonal planar, so the trigonal planar molecule makes a 120⁰ bond angle concerning the central atom. Again, the hybridization value also confirmed the bond angle value.

• The bond angle is now predicted from the hybridization value of the central atom.
• The bond angle formula according to Bent’s rule is COSθ = s/(s-1).
• The central atom Al is sp² hybridized, so the s character here is 1/3^rd
• So, the bond angle is, COSθ = {(1/3)} / {(1/3)-1} =-( ½)
• Θ = COS^-1(-1/2) = 120⁰
• There is no deviation factor present so the bond angle is perfectly 120⁰.

7.      AlI₃ lewis structure formal charge

The formal charge is a theoretical concept that can predict the presence of the charge over a particular atom of a molecule. Let’s find out the formal charge of AlI₃.

The value of the formal charge for the AlI₃ molecule is zero because the +3 charge of Al is satisfied by the -1 charge of each Iodine. For the calculation of the formal charge of the AlI₃ molecule at first, we should assume the same electronegativity for Al and iodine atoms.

• The formal charge of the AlI₃ can be calculated by the formula, F.C. = N v – N l.p. -1/2 N b.p
• The formal charge accumulated by each Al atom, 3-0-(6/2) = 0
• The formal charge accumulated by each N atom, 7-6-(2/2) = 0
• So, the overall formal charge by the three Ca atoms and two N atoms are 0 + 0*3 = 0

8.      AlI₃ hybridization

Different orbitals have different energy so they undergo hybridization to form a new hybrid orbital to form a bond. Let’s discuss the hybridization of AlI_{3 in} detail.

The hybridization of central Al in AlI₃ is sp² which can be predicted from the following table.

• We can calculate the hybridization by the convention formula, H = 0.5(V+M-C+A),
• So, the hybridization of central Al is, ½(3+3+0+0) = 3 (sp²)
• One s orbital and two p orbitals of Al are involved in the hybridization.
• The lone pairs of Iodines are not included in the hybridization.

9.      Is AlI₃ solid?

Solid is a physical state of matter having a higher density and retains its shape and all atoms are tightly bound. Let’s see if AlI₃ is solid or not.

AlI₃ is solid because its crystal lattice consists of a monoclinic structure. The hydrated crystal also presents within the lattice structure making it strong and ductile. In the dimeric form, two Al centers are surrounded by the six iodines very tightly making van der Waal’s force of attraction higher.

AlI₃ is a white powder at room temperature. But its dimeric form is stronger and cannot be cleaved in lower energy. The density of the molecule is greater than 3 gm/cm³.

10. Is AlI₃ soluble in water?

When a molecule is soluble in water then it gets dissolved in water and breaks its bond in the aqueous solution. Let us check whether AlI₃ is soluble in water or not.

AlI₃ is soluble in water because the hydrated part present within the molecule attached water molecule and gets soluble in water. Also, in the presence of a hydrated part, the hydration energy for this molecule is very high.  Also, the Al³⁺ can attract water molecules very easily.

11. Is AlI₃ polar or nonpolar?

The presence of a permanent dipole moment makes a molecule polar for this reason some physical characteristics will be changed. Let’s see if AlI₃ is polar or not.

AlI₃ is not polar because the shape of the molecule is trigonal planar which is symmetrical, so the direction of dipole-moment is equal and opposite to each other so they cancel out one another, so, the value of the resultant dipole-moment value is zero which makes the molecule non-polar.

12. Is AlI₃ a molecular compound?

A molecular compound is one kind of covalent compound formed by satisfying the valency of individual atoms. Let’s see if AlI₃ is a molecular compound or not.

AlI₃ is a molecular compound because the valency of the cation and anion is fully satisfied here. The stable valency of Al is 3 which is satisfied by the three mono valent iodine atoms. So, on the electrolysis it is formed Al³⁺ and 3I^–, because to maintain the valency three iodines are required.

The molecule has a fixed ratio of the cation and anions and also in the molecule stoichiometric count is fixed.

13. Is AlI₃ acid or base?

As per Arrhenius’s theory if a substance release H⁺ or OH^– in an aqueous solution is known as acid or base respectively. Let’s see whether AlI₃ is acid or base.

According to the Arrhenius theory AlI₃ neither be acid nor be base, because it cannot release H⁺ or OH^– in an aqueous solution as there is no presence of such ions. But on the acceptance of electrons or donation of electrons, we can be categorized as lewis acid or base.

AlI₃ is a strong lewis acid because it can accept electron density as Al is not complete in its octet so it can accept electron in its vacant orbital easily, also there are three electronegative iodines present which can draw electron density from the Al center and makes the molecule strong lewis acid.

14. Is AlI₃ electrolyte?

Electrolytes are that substance can be ionized an in aqueous solution and carry a charge through the solution. Let’s check if AlI₃ is an electrolyte or not.

AlI₃ can behave as electrolyte because it can be ionized as Al³⁺ and 3I^–, so those charged particles can easily carry electricity through the solution. So, it can behave as an electrolyte. But the nature of AlI₃ is a strong electrolyte which can be discussed in the next part of the article.

AlI₃ can be completely ionized in the aqueous solution to form Al³⁺ and I^–. The mobility and charge potential of Al³⁺ is very high because it is one of the most electropositive atoms.

15. Is AlI₃ salt?

Salt consists of cations other than H⁺ and anions other than OH^– and bonded with an ionic bond. Let’s check whether ALI₃ is a salt or not.

AlI₃ is a salt because it has cation Al³⁺ and anion I^– which are other than H⁺ and OH^–. Also, there is ionic interaction occurred because the charge of the cation and anions are very high. Although it is lewis acid sometimes it behaves as salt, the main reason it is a strong electrolyte in nature.

Only salt can carry electricity in an aqueous solution because it can be ionized in the solution and behaves as an electrolyte, so, AlI₃ is salt.

16. Is AlI₃ ionic or covalent?

According to Fajan’s rule, no molecule is pure ionic or covalent it depends on the theory of polarizability.

AlI₃ is a covalent molecule because it can share electrons in the bond. The central atom undergoes sp² hybridization and the only covalent molecule can undergo hybridization. The ionic molecule is made by lattice crystal structure which is also shown in the AlI₃, so it can be thought of as ionic.

The cation Al³⁺ has a higher ionic potential (charge density high) so it can be polarized by many anions. Again, the polarizability of iodide is very high due to its larger size and can be easily polarized by the cation and developed ionic character.

Conclusion

AlI₃ is a non-polar covalent molecule but also shows ionic characteristics. It exists as a dimer to complete the octet over the Al center. It acts as lewis acid and a strong electrolyte.

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Biswarup Chandra Dey

Hi……I am Biswarup Chandra Dey, I have completed my Master’s in Chemistry from the Central University of Punjab. My area of specialization is Inorganic Chemistry. Chemistry is not all about reading line by line and memorizing, it is a concept to understand in an easy way and here I am sharing with you the concept about chemistry which I learn because knowledge is worth to share it.