Hybridization of I₃⁻

What Is Triiodide?

Triiodide in Chemistry usually refers to the triiodide ion, I3- This anion, one of the polyhalogen ions, is composed of 3 iodine atoms and is formed by combining the aqueous solution of iodine and iodide salts. A few salts of the anion have been isolated, including ammonium triiodide ([NH4]+[I3] and thallium(I) triiodide (Tl+[I3])). The triiodide is recognized to be red colour in solution.

What Is The Hybridization Of Triiodide Ion?

To know the hybridization of any molecule, there is a simple formula that can be used. This formula is used to find out the hybridization number, which helps in knowing the molecule hybridization.

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The hybrid numbers and the hybridization that it determines are listed as follows.

  • 2: sp hybridization

  • 3: sp2 hybridization

  • 4: sp3 hybridization

  • 5: hybridization

The formula to find the number is given by,

Hybridization Number = valence electron number + monovalent + (negative charge) – (positive charge)/2

The iodine atoms' valence electrons will be 7 because there are 7 electrons in the outermost shell. The number of the monovalent atom is 2 because the two out of three iodines are monovalent. Now, coming to the charge of the I3- ion, it has a negative charge. So, this negative charge value will be 1.

Now if we place all the values according to the formula, it becomes,




Thus, the hybridization number is 5, which means it is sp3d hybridized.

Another way to find the hybridization of a given molecule is by taking the help of lone pairs and valence electrons. Here, the number of lone pairs in the molecule is 3 and the number of atoms sharing valence electrons is 2.

Henceforth, 3+2=5, which also determines the same sp3d hybridization.

Name of the Molecule

Triiodide Ion

Molecular Formula


Hybridization Type


Bond Angle




Lewis Structure

Lewis structure is nothing but the electron representation of the molecules. There are lone pairs and valence electrons that help in determining the shape and hybridization of the molecule. As there are Iodine molecules, one molecule of iodine will present in the centre. Iodine also lies in the seventh group of the periodic table and has 7 valence electrons in its outer orbit.

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Here, we have 3 molecules of iodine, which along with an extra electron that gives it a negative charge. So, the total valence electrons count is given by 7×3 + 1= 22.

Moreover, in this molecule, there are 22 valence electrons in total. Now, there is an octet rule followed by every atom. According to this rule, every atom should have 8 electrons in their outer orbit. So, if there are 8 electrons in the central atom's outer shell, there exist 2 other atoms required to complete their octet. As all the atoms need 8 electrons in their outer shell for octet completion, 1 central iodine's electron atom will be taken by both neighbouring iodine atoms. It means 8-1-1=6 because both the atoms will take the electrons.

So, the valence electrons on the central atom of iodine will become 6. These 6 electrons will form the electron lone pairs that do not bond. As it forms the pair of electrons now, there will be 3 lone pairs, and 2 bond pairs of the electrons because each Iodine atom has a bond with the central atom sharing 1 electron each. So, in total, there are 3 lone pairs and 2 bond pairs on the central atom.

I3 Molecular Geometry and Bond Angles

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The molecular geometry of I3- is linear. While there are 3 Iodine atoms, one of the atoms has a negative charge, which further gives 2 bond pairs and 3 lone pairs of electrons. Its steric number will be 5. The 3 lone pairs will repel each other and take up the equatorial positions. The remaining 2 Iodine atoms present at 180° from each other.

Polarity Of I3- Ion

So, there is a tricky part of this ion here. Firstly, we can call the charge present on it as a polyatomic ion instead of a molecule. Ions are the charges that we see on the molecules. As I3- has only 1 electron, this Ion has an overall negative charge. Whereas, the molecules have polarity because it has both the charges which are charged partially positively and negatively. There exists a dipole moment on the molecules based upon the charges separation on the molecule. If the distance between both charges is large, then the dipole moment will also become larger.

However, when we discuss II3- ion, it is a negatively charged ion. Even while drawing its Lewis structure, we do not see any dipole moment of the polar bonds in it, because the overall charge itself is negative on the ion. So, it is neither polar nor nonpolar. However, if we have to describe the ion, we can use the phrase "like a polar molecule" because I3- is soluble in water.

FAQs (Frequently Asked Questions)

1. Explain the Shape of an I3- Molecule.

Ans: Shape of I3- Ion

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The shape of an I3- the molecule is Linear. There exist 3 Iodine atoms out of which 1 has an extra negative charge. Due to this, one extra electron, there are 3 lone pairs of electrons and 2 bond pairs, making its steric number as 5. As there are 3 lone pairs on the central Iodide atom, they try to repel each other as much as possible. So, to be away at a greater distance, the pairs take the equatorial positions, and the remaining two Iodine atoms are 180° from one another. Therefore, the overall shape of the I3- ion results linearly.

2. Explain the Properties of Triiodide ion.

Ans: Let us address some properties of the triiodide molecule.

  • The triiodide ion is the simplest polyiodide, from where several higher polyiodides exist. It appears yellow in low concentrations and as brown at higher concentrations in solutions. The triiodide ion is responsible for the blue-black colour, a well-known one that arises when iodine solutions interact with starch. Iodide does not react with starch, nor do solutions of iodine in the nonpolar solvents.

  • Lugol's iodine contains a stoichiometric amount of elemental iodine and potassium iodide so that significant amounts of triiodide ion contain this solution.

  • The iodine Tincture, nominally a solution of elemental iodine in ethanol although, it also contains significant amounts of triiodide because of its content of both iodide and water.