Is Aluminium chloride (AlCl3) ionic or covalent? Types of bond in AlCl3

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Aluminum chloride is composed of aluminum and chloride also known as aluminum trichloride or aluminum (III) chloride having the chemical formula AlCl3. It appears as white to grey powder with a pungent odor. In solution forms, it appears as a straw-colored liquid. It is mostly used in the production of aluminum metals.

In this article, we will discuss Is Aluminum chloride (AlCl3) is ionic or covalent? Bond types in aluminum chloride, etc.

So, Is AlCl3 ionic or covalent? AlCl3 is a covalent compound as the bond formed between one aluminum and three chlorine atom through the mutual sharing of electrons because of the polarization power effect(Based on Fajan’s rule). Also, the difference in electronegativity value between aluminum and a chlorine atom is not big enough to make the ionic bond between them according to the Pauling scale.

Although, AlCl3 is not purely covalent in nature, there is a partial ionic character as well.

Let’s read in detail, why AlCl3 is a covalent compound? How covalent bonds formation takes place in AlCl3?

Name of MoleculeAluminium chloride
Chemical formulaAlCl3
Molar mass133.34 g/mol
Bond typeCovalent
Percentage character55% covalent and 45% ionic character

Why AlCl3 is an covalent compound?

A covalent compound is a chemical compound that is made when two nonmetals or two p-block elements together share their valence electron with each other to complete the octet and attains stability.

Examples of some compounds that form covalent bonds – H2O, H2S, NH3, NO2, SO2, etc.

Also Read:

NoteIt is not necessary always that a covalent bond will form between two nonmetals, it can also be formed when one atom is metal and another is a nonmetal.

AlCl3 is a covalent compound because aluminum and three chlorine atoms are linked through the sharing of electrons with each other, although, aluminum is a metal and chlorine is a nonmetal, still, there does not occur a complete transfer of electrons between the metal and the non-metal. This can be best explained by the Fajans‘ rule.

Fajan rule’s are used to predict whether a chemical bond will be covalent or ionic, and depend on the charge on the cation and the relative sizes of the cation and anion.

Based on the Fajan rule, the ionic or covalent compounds can be distinguished by the following characteristics.

Ionic characteristicsCovalent characteristics
Small anionLarge anion
Low chargeHigh charge
Large cationSmall cation

Point to remember while using the Fajans‘ rule to predict the covalent or ionic character:

  • The Fajans’ rule states that a compound with a large anion, large charge, and small cation will adopt the covalent character whereas a compound with a small anion, small charge, and large cation will adopt an ionic character.
  • Fajans’ rule depends on the size of the relative size of cation and anion for predicting the ionic or covalent character in the molecule.
  • The smaller the size of the cation and the larger the size of the anion, the greater is the covalent character in the molecule.

Most important: The cation with a high positive charge will attract the electron in the outermost shell of the anion towards itself such that the electron cloud of the anion is distorted, hence, the electron is drawn into the space between positive cation and negative anion, and these electrons get shared between cation and anion. Therefore, the compound exhibit a covalent character.

Fajan rule of covalent character in AlCl3

Now, we will try to know Why AlCl3 has a covalent character according to the Fajans’ rule.

  • AlCl3 has two ions, Al3+ as cation and Cl as the anion. The Al3+ is very small and highly charged, therefore, has a high charge density. The Cl is a very large anion compared to Al3+ and has a very low charge density.
  • Al3+ has a very high polarizing power since it is highly charged, so, when it comes close to the Cl, then it polarizes the electron cloud of Cl to the large extent.
  • After polarizing the electron cloud of Cl by the Al3+, the symmetrical shape of the Cl anion get distorted and electrons are drawn into the space between Al3+ and Cl.
  • The electrons that are drawn into the space between cation(Al3+) and anion(Cl) are gets shared to complete their octet, hence, the bond formed between the ions of AlCl3 is exhibited great covalent character.

In short, “Al3+ is highly charged in nature and it can polarise the electron clouds of Cl to a large extent. So, electrons get shared between the two ions. Hence the AlCl3 compound is a covalent one”

why is AlCl3 covalent compound and not an ionic compound.

Although, the AlCl3 is formed from metal and nonmetal and many of you assumed that it is an ionic compound but the bond formation in these types of compounds such as AlCl3, AlF3, AlBr, etc are best explained by the Fajans’ rule.

Ionic character in the covalent compounds and covalent character in an ionic compound can easily be predicted by the Fajans’ rule.

How covalent bond is formed in AlCl3?

The covalent bond is formed in AlCl3 because each chlorine atom gets 1 electron from the Al atom through the sharing and attains stability.

Let’s see how covalent bond formation takes place in AlCl3.

There are two types of atoms present in the AlCl3 molecule – aluminum and chlorine atom.

⇒ The aluminum atom is a metal that belongs to Group 3 in the periodic table and has three valence electrons in the outermost shell.

valence electron in aluminium atom

⇒ The chlorine atom is also a nonmetal that belongs to Group 7 in the periodic table and has a 7 valence electron in its outermost shell.

valence electron in chlorine atom

So, when the cation(Al3+) and anion(Cl) of the AlCl3 compound come closer to each other, the cation(Al3+) polarize the electron cloud of the anion(Cl), hence, the shape of the anion get distorted and electron is shared between these ions, and a covalent bond is formed in AlCl3 compound.

covalent bond formation in AlCl3

As we see in the above picture of covalent bond formation in AlCl3, the chlorine atom has 7 valence electrons in its outermost shell and it needs 1 more to complete the octet, hence, each chlorine atom gets 1 electron from the Al atom through the sharing and attains stability

Note: An aluminum atom in the AlCl3 compound attains stability by just having 6 valence electrons around it because it is exceptional to the octet rule.

Why is AlCl3 not a ionic compound?

An ionic compound is a compound that involves the exchange of valence electrons between the atoms usually electron is a transfer by metal and acceptance by non-metal.

Examples of some ionic compounds – MgO, Na2O, MgCl2, NaCl, etc.

Also Read:

AlCl3 molecule has more covalent character than the ionic character hence, it is said that AlCl3 is a covalent compound instead of ionic.

According to the Pauling scale, the AlCl3 molecule is not an ionic compound because the difference in electronegativity value between aluminum and a chlorine atom is not big enough to make an ionic bond between them.

According to the Pauling scale of electronegativity-

  • The covalent bond is formed between the two atoms when their electronegativity difference occurs less than 1.7.
  • The ionic bond is formed between the two atoms when their electronegativity difference occurs more than 1.7.

pauling scale for chemical bonds

Now check the electronegativity difference in AlCl3 molecule-

⇒ The electronegativity value for aluminum atom = 1.61

⇒ The electronegativity value for chlorine atom = 3.16

∴ The difference of the electronegativity between aluminum and chlorine atoms = 1.55

So, the difference in electronegativity in the Al-Cl bond is less than 1.7, hence, it is low enough to make a covalent bond between them as per the Pauling scale of electronegativity.

how many covalent bonds in AlCl3

∴ AlCl3 compound has three single covalent bonds(Al-Cl) as shown in its lewis structure.

Also, checkHow to draw the lewis structure of AlCl3?

Is AlCl3 pure covalent compound?

A pure covalent bond is mostly formed in the molecule have has the same type of atoms and the same electronegativity, for example – O2, F2, Cl2, etc.

Basically, a homonuclear molecule has the ability to form a pure covalent bond because the electronegativity of atoms is the same, hence, sharing of electrons is also equal in them.

In short, a pure covalent bond is formed between the atoms when they share an equal number of electrons.

So, Is AlCl3 a pure covalent compound? No, AlCl3 is a not pure covalent compound because it is a heteronuclear molecule that contains different types of atoms (aluminum and chlorine atom), and their electronegativity is also not the same.

The electronegativity for aluminum is 1.61 whereas, for chlorine, it is 3.16. So, the electrons are more shared towards the chlorine atom as it is higher electronegative than the aluminum atom, hence, unequal sharing of electrons occurs between aluminum and chlorine atoms.

So, the bonds formed in the AlCl3 molecule are not a pure covalent bond, hence, AlCl3  is not a pure covalent compound.

AlCl3 is not a pure covalent compound

Are the bonds in AlCl3 polar covalent or nonpolar covalent?

A polar covalent bond is formed when the unequal sharing of electrons occurs between two atoms because of their difference in electronegativity value.

When the difference of electronegativity occurs between two atoms lies in between 0.4 to 1.7, then the bond formed between two atoms is polar in nature according to the Pauling scale of electronegativity.

polar covalent bond

A nonpolar covalent bond is formed when the equal sharing of electrons occurs between two atoms because of the very little difference in their electronegativity value.

When the difference of electronegativity occurs between two atoms is less than 0.4, then the bond formed between two atoms is nonpolar in nature according to the Pauling scale of electronegativity.

nonpolar covalent bond

So, Are the bonds in AlCl3 polar covalent or nonpolar covalent? The Al-Cl bond in the AlCl3 molecule is a polar covalent because the electronegativity difference between aluminum and a chlorine atom is 1.55 which is in the range of the Pauling scale of the polar covalent bond from 0.4 to 1.7.

Also, the unequal sharing of electrons occurs in AlCl3 molecules because chlorine(3.16) is higher electronegative than an aluminum atom(1.61), hence, it will pull more electrons towards itself, therefore, the chlorine atom induced a negative charge on it and aluminum atoms get a slightly positive charge.

The separation of charges generates the dipole moment directed from positive to negative, hence, due to all these a bond is formed between aluminum and chlorine, which is called a polar bond.

polar covalent bond in AlCl3

Properties of aluminum chloride

  • It has a molar mass of 133.341 g/mol.
  • It appears as white or pale yellow solid.
  • It has a melting point of 180 °C.
  • It has a low melting and boiling point.
  • It is non-flammable and powerful lewis acid.
  • It has a coordinate geometry of octahedral in the solid phase and tetrahedral in the liquid phase.

FAQ

Why is AlCl3 covalent while AlF3 is ionic?

As Fajans‘ rule said,  the covalent character increase when the size of the cation is small and the size of the anion large, so that, the anion easily gets polarized by the cation, and electrons get shared between them.

So, in the case of AlCl3 and AlF3, the cation(Al3+) is the same for both compounds, hence, just ignore it. Now see the size of the anion of both compounds.

Clearly, Size of Cl > size of F

Hence, chlorine gets easily polarised by the Al3+ but on the other hand, the fluorine size is so small that Al3+ is not able to polarize it to such a great extent.

Therefore, we can say, due to the size difference of anion, the AlCl3 is covalent while AlF3 is ionic.

Why AlCl3 is covalent and not an ionic compound?

If you only see that AlCl3 is made up of one metal and nonmetal, then you might assume it as an ionic compound, and here’s what you get totally wrong.

AlCl3 has a predominately covalent character as described by the Fajans‘ rule. Although the AlCl3 compound is not purely covalent, it has some ionic character as well but it is predominately by the covalent character.

The AlCl3 bonds have 55% covalent character and 45% ionic character, hence, it is categorized as a covalent compound instead of the ionic compound.

Here, you can find the ionic percentage character of any compound by just placing the electronegativity of atoms.

Summary

  • Is Aluminum chloride (AlCl3) ionic or covalent? AlCl3 (aluminum chloride) is a covalent compound because cation(Al3+) is highly charged and able to polarize the electron cloud of the anion(Cl) easily to the large extent, this phenomenon has drawn the electrons into the space between the positive and negative ion and here, these electrons get shared, and makes the covalent bond.
  • AlCl3 bonds have 55% covalent character and 45% ionic character, hence, it is not a pure covalent compound, it exhibits some ionic character as well.
  • The difference in electronegativity between aluminum (1.61) and chlorine(3.16) is 3.16–1.61 = 1.55 which is lower than 1.7, hence, according to the Pauling scale, the bond formed between aluminum and a chlorine atom is covalent in nature.
  • There are three Al-Cl covalent bonds present in the AlCl3 molecule.
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