Nitrogen triiodide (NI3) lewis dot structure, molecular geometry, hybridization, polarity

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Nitrogen triiodide appears as purple gas is an inorganic compound having the chemical formula NI3. It releases a purple cloud of iodine vapor when touched gently because of its extremely sensitive contact explosion property. 

In this tutorial, we will study NI3 lewis structure, molecular geometry, bond angle, polarity, hybridization, etc.

Properties of Nitrogen triiodide

  • It appears as purple gas.
  • It is not soluble in water but soluble in organic solvents such as diethyl ether.
  • It sublimes at −20 °C.
  • It has complex structure chemistry.
  • It has a molar mass of 394.7 g/mol.
Name of MoleculeNitrogen triiodide
Chemical formulaNI3
Molecular geometry of NI3Trigonal pyramid
Electron geometry of NI3Tetrahedral
Bond angle107.1º
Total Valence electron for NI326
The formal charge of NI30

How to draw lewis structure for NI3

NI3 lewis structure contains three N-I bonds, nitrogen in center position whereas all three iodine atoms are at the terminal position. There is only one lone pair present on the central atom in the NI3 lewis structure.

Each iodine atom at the surrounding position contains three lone pairs and is connected to the central atom with a single bond.

Let’s see how to draw the lewis structure of NI3 with a good approach.

Follow some steps for drawing the lewis structure of NI3

1. Count total valence electron in NI3

Valence electrons are usually the electrons found in the outermost shell of an atom. To know the total valence electron in the NI3 molecule, we get to know the valence electron of an individual atom(nitrogen and iodine).

The nitrogen atom belongs to the 15th or 5A group in the periodic table, therefore, the electrons present in its outer shell are 5 whereas the iodine atom is a member of the halogen family and has 7 electrons in its outer shell.

⇒ Total valence electron in Nitrogen = 5

⇒ Total valence electron in Iodine = 7

∴ Total valence electron available for drawing the NI3 lewis structure = 5 + 7*3 = 26 electrons     [∴NI3 contains one nitrogen and three iodine atoms]

2. Find the least electronegative atom and placed it at center

The atom with less electronegativity is more prone to share the electrons than the atom with higher electronegativity. The electronegativity of nitrogen and iodine atom is very near to each other, so, the other method to determine the central atom of the lewis diagram is by observing the least numerous element in the compound.

The element which repeats least in the compound should be the central atom in the lewis diagram. So, in the case of NI3, the nitrogen atom is repeated only one time whereas the iodine atom repeats three times.

Hence, put the nitrogen atom at central position whereas spread the three iodine atom around it.

3. Connect nitrogen and iodine atoms with a single bond

Here, we will join together each iodine atom to the nitrogen central atom via a single bond.

Now we need to know how many valence electrons we used in the above structure before moving to the next step. As we see in the above structure, there is three single bond used and one single bond made up of two valence electrons, hence, (3×2) = 6 valence electrons are used in the above structure from a total of 26 valence electrons for NI3 lewis structure.

∴ (26 – 6) = 20 valence electrons

Now we are left with 20 valence electrons more.

4. Placed remaining valence electrons starting from outer atom first

The outer atom in the NI3 molecule is iodine that needs 8 electrons to complete its octet and attains stability. Let’s try to put the remaining valence electrons around each iodine atom till they complete their octet.

By putting 6 valence electrons represented as dots around each iodine atom in the above structure, we see that they completed their octet comfortably as all three iodine atoms have 8 electrons(6 electrons represent as dots + 2 electrons in forms of single bond) in its octet shell.

Now, we have only two valence electrons left as in the above structure, we use (three single bonds that contains 6 electrons + 18 electrons as dots)

5. Complete central atom octet  and use covalent bond if necessary

So, we still have 2 valence electrons in our pocket, and nitrogen as a central atom also needs 8 electrons to complete its octet but it already sharing 6 electrons through the three single bonds connected to the iodine atoms.

Hence, nitrogen needs only 2 electrons to complete its complete, therefore, put the 2 remaining valence electrons on it.

NI3 lewis structure

So, in the above structure, each atom(iodine and nitrogen) completed its octet and we also used all the valence electrons that are available for drawing the lewis structure of NI3.

What are the electron and molecular geometry of NI3?

The molecular geometry of NI3 is trigonal pyramidal, and electron geometry is tetrahedral because the lone pair present on the central atom creates repulsion between adjacent bonded pairs of electrons, as a result, two iodine atoms in equatorial position pushes far apart giving its molecular geometry the same as a trigonal pyramid.

The generic formula for NI3 is AX3N1. So, according to the VSEPR chart, if the molecule has the formula of AX3N1  then the molecule shape of that molecule is trigonal pyramidal, and electron geometry is tetrahedral.

Bonded atomsLone pairGeneric formulaHybridizationMolecular geometryElectron geometry
30AX3Sp²Trigonal planarTrigonal planar
21AX2NSp²BentTrigonal planar
12AXN2Sp²LinearTrigonal planar
31AX3N1Sp³Trigonal pyramidTetrahedral
32AX3N2Sp³dT-shapedTrigonal bipyramidal

Hybridization of NI3

The hybridization of NI3 can easily find out by knowing the steric number of its central atom.

The steric number is an addition of bonded atoms to the central atom and the presence of lone pair on that central atom.

∴ Steric number of NI3 = (Number of bonded atoms attached to nitrogen + Lone pair on nitrogen)

As we see in the lewis structure of NI3, there are three iodine atoms bonded to the nitrogen central atom and only one lone pair present on the nitrogen central atom.

∴ Steric number of NI3 = (3 + 1) = 4

The steric number of 4 implies the hybridization state in the NI3 molecule is Sp3.

The bond angle of NI3

“The ideal bond angle for the Nitrogen tribromide is 109.5° since it has a Trigonal pyramidal molecular geometry. Experimentally we would expect the bond angle to be approximately Less than 109.5°.”[check here]

Nitrogen triiodide polarity: is NI3 polar or nonpolar?

NI3 is a polar molecule because the one lone pair present on the nitrogen central atom generates repulsion between electrons pairs that makes the overall structure bent and this creates unequal charge distribution of charge within the structure that generates a permanent dipole moment.

Let’s understand it with the help of three factors. (a). Electronegativity (b). Dipole moment (c). Molecular shape

Three factors that indicate the polarity of NI3 

1. Electronegativity: 

Electronegativity means the tendency of an atom to attracting electrons towards itself. If the electronegativity difference between the atoms is high then the polarity will also be higher. Now, look at the electronegativity of nitrogen and iodine. 

The difference between the electronegativity of nitrogen and iodine is directly proportional to the Polarity of the NI3 molecule.

The electronegativity of nitrogen is 3.04 and for iodine, it is 2.66. And the difference of electronegativity between nitrogen and iodine is 0.38 which is very little.

So, the N-I bond is non-polar due to less electronegativity difference among atoms but the overall molecule is polar due to the presence of lone pair on the central atom of the NI3.

2. Dipole moment

Dipole moment ensures the strength of polarity between nitrogen and iodine atoms. As greater the dipole moment of the molecule, the more is the polar nature of that molecule. 

The dipole moment of NI3 is very less but still, it is non-zero that causes NI3 to become polar in nature.

In mathematical terms, dipole moment can be expressed as-

⇒ Dipole moment formula =  charge on the atoms * the distance between them 

∴ D = Q × R

3. Geometrical or molecular shape

The geometrical shape is also a big factor to determine is NI3 polar or non-polar.

As we know the molecular shape of NI3 is trigonal pyramidal which is asymmetric and when the shape of the molecule is asymmetric or distorted it doesn’t cancel the dipole moment across the molecule. Hence cause polar nature of the molecule

All these factors are enough to understand why NI3 is polar in nature.


How many bond pair and lone pair a central atom has in NI3 lewis structure?

In the NI3 lewis structure, only one lone pair is present on the nitrogen central atom whereas three bonded pairs are attached to the outer atom(iodine).

Why NI3 molecular geometry is the trigonal pyramid and its electron geometry is tetrahedral?

Always remember, molecular geometry only considers bond pairs of electrons to determining the shape of any molecule, and electron geometry considers both bond pair and lone pair to determine the geometry of any molecule.

As NI3 contains one lone pair and three bonded pairs. So, according to the VSEPR chart, the electron geometry of NI3 is tetrahedral while molecular is trigonal pyramidal.

Is the lewis structure of NI3 the same as NF3 and NCl3?

Yes, the lewis structure of NI3 is almost the same as NF3 and NCl3.

NF3 lewis structure has 3 fluorine and 1 nitrogen atom connected with three single bonds and NCl3 lewis structure has 3 chlorine and 1 nitrogen connected with three single bonds also.

In all these molecules(NI3, NF3, and NCl3), there is one lone pair present on the central atom.


Now it’s time to take a quick review of an overall article with the help of some bulleted points.

  • The total valence electron available for the NI3 lewis dot structure is 26.
  • The hybridization of NI3 is Sp³.
  • Nitrogen triiodide is slightly polar in nature.
  • The molecular geometry of NI3 is trigonal pyramidal and its electron geometry is tetrahedral.
  • Lewis structure of NI3 contains 1 lone pair and 3 bonded pairs. 
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