I3- Lewis Structure: Drawings, Hybridization, Shape, Charges, Pairs.

In this article, we shall learn about I3– Lewis-structure drawings, hybridisation, shape, charges, pairs and how to construct it following octet rule and some facts and detailed explanantion about the polyatomic ion which holds great importance in industrial sector.

I3- Lewis structure or lewis dot structure is a simple representation of the electronic structure of a molecule that briefs about the number of bonds formed, number of bond pairs required to fulfill the octet rule and lone pairs available.

The Lewis theory is based on octet rule which states that an atom always tend to arrange 8 electrons around themselves to acquire a stable or a noble gas configuration. There are, however, some exceptions like when a molecule is electron deficient; when it has odd number of electrons; or molecules that has extra electrons in their valence shells. E.g., BH3 ,SF6 , H2, NO etc. I3- lewis structure is one of them.

I3 structure from wikipedia

Methods to draw a Lewis Structure :

Note: Elements having expanded valence shells like 3d elements, it can exceed the octet rule like SF6 , PFor elements with fewer valence electrons can have incomplete octet like H2 .

How to draw a I3 Lewis structure ? :

  1. Iodine belongs to 17th group and 5th period. It has 7 valence electrons, with expanded shells to occupy any extra electrons apart from the 8 electrons. It has 3 same I atoms with same electronegativity, therefore, choose any one as the central atom. It has a total of 22 valence electrons from 3 I atoms and a negative charge.
  2. Draw a covalent bond between each atoms with the central atom to fulfill the outer orbit. In doing so, we get 3 lone pairs of electrons on the central atom and 2 lone pairs of electrons each on the surrounding atoms. As I has empty 4d shells it can expand to accommodate extra electrons thereby violating octet rule which is generally observed for heavier elements. This gives the I3 lewis structure.
I3 Lewis structure

I3 Lewis structure Formal Charge :

Now, we have to assign their formal charge to obtain the complete stable I3– lewis dot structure.

It briefs about the electronic charge of each atom in a molecule based on the Lewis dot structure.

Generally, formal charge can be calculated mathematically by the formula :

Formal charge = (Number of valence electrons in a free atom of the element) –  (Number of unshared electrons on the atom) – (Number of bonds to the atom)

In addition, Charge on the molecule= sum of all the formal charges.

Lewis structure of I3
Each atom is assigned its respective formal charge which will give the overall charge of the ion.

Formal charge of Ia = 7-6-1 = 0 Formal charge of Ib = 7-6-2 = -1    

I3 Lewis structure resonance  :

Resonance structure I3 Lewis structure

Dative or coordinate bonds are formed by sharing two electrons covalently by single atom to the nearest neighbouring atom.

The two I atoms at the end are similar due to resonance. It avoids forming any double bonds even though it has extra subshells to accommodate electrons is because it is most stable when it acquires a linear shape to prevent steric angular strain as the single bonds do not repel each other greatly than the single bond when present in a double bond. Therefore, this form is the most stable and likely resonance structure of I3lewis structure.

I3 Lewis structure valence electrons :

i3- lewis structure
Bond represents bond pairs. The dots represents lone pairs.

Electronic configuration of I : [Kr]4d105s25p5. Its valence electrons are 5s25p5 which counts to a total of 7 outermost electrons. I3 Lewis structure has a total of 22 valence electrons. It has 2 bond pairs ( that formed a single covalent bond here).

Note: In actual sense of chemistry, there is a coordinate bond formation between I2 and I ( electron pairs shared completely by iodide ion, a type of covalent bond).

I3 Lewis structure lone pairs :

I3 Lewis structure has a total of 9 lone pairs of electrons that did not participate in bond formation and residing on respective I atoms.

I3 Lewis hybridisation :

There is a simple rule or equation to be followed to find out the hybridization of molecules real quick.

Hybridisation of a molecule = ( Valence electrons of the central atom + Number of monovalent atoms attached to the central atom + Negative charge on the molecule – Positive charge on the molecule )/2

Here, I3 Lewis structure hybridization = (7+2+1)/2 = 5 i.e., sp3d.

I3 Lewis structure shape :

I3 lewis structure has a trigonal bipyramidal geometry which is well justified as the central atom contains 3 lone pairs of electrons which can stay at maximum distance with less repulsion when they occupy the equatorial position at an angle of 1200 with each other. The other two bond pairs i.e., the two I atoms occupy the apical positions. Therefore, the shape of I3 lewis structure is linear.

The I atoms in apical position are at 1800 to each other.

I3 Lewis structure angle :

The I3 Lewis structure adopts a linear shape which has an angle of 1800 .

I3- Lewis Structure
The angle between two I atoms is 1800 .
MoleculeI3, Triiodide ion
TypePolyatomic ion
Hybridisation, Geometrysp3d , Trigonal Bipyramidal
Bond and Lone pairs2 , 9
Table: Chracteristics of I3

I3 Lewis structure octet rule :

I3 Lewis structure violates octet rule as it has expanded 4d shells which can accommodate extra electrons besides the 8 electrons required to fulfil the octet rule as evident from its lewis dot structure.

Is I3 stable ?

 Yes, triiodide ion is found to be stable.

As iodine element belongs to 5th period, it has empty 5d shells which can expand its octet to accommodate the extra electron pair provided by the iodide ion, I to I2 . They have been found to exist in aqueous solution as well as in crystalline form with a variety of cations.

Is I3 ionic or covalent ?

Triiodide ion is a polyatomic ion with an overall negative charge in one end of the linear molecule. Hence, by default it is going to be ionic. There might be some questions regarding its polarity as its dipole each other being linear in shape, but there is always some negative charge residing due to which it has some dipole moment associated with it. Also, a polyatomic ion will never exist as a free ion in a solution , its opposite charged ions will surround it always. In solid state, there is a slight deviation from linearity giving some dipole moment.

Even though the dipoles cancels each other, some negative charge is still left.


  1. It is widely used as a redox couple in dye synthesized solar cells widely known as DSC.
  2. It is used as a conducive materials in solar panels, batteries, electrochemical cells.
  3.  It is used as an interesting ion in electrical and magnetic materials, in host-guest compounds etc.
  4. It is used for controlled reactions, as an indicator in chemical redox reactions, for stain removal of certain dyes produced during reactions.

Conclusion :

Triiodide ion, I3- Lewis structure , is a linear polyatomic ion with sp3d hybridisation and trigonal bipyramidal geometry and acquiring a linear shape known through VSEPR model.

Nandita Biswas

Hello. I am doing my Masters in Chemistry with a specialization in organic and physical chemistry. Also, I have done two projects in chemistry- One dealing with colorimetric estimation and determination of ions in solutions. Others in Solvatochromism study of fluorophores and their uses in the field of chemistry alongside their stacking properties on emission. I have also specialized in Resin chemicals. Let's connect through LinkedIn-https://www.linkedin.com/in/nandita-biswas-244b4b179

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