Nitrogen trichloride is a pungent-smelling halogen nitride represented by the chemical formula NCl3. It appears as an oily-yellow liquid at room temperature. The typical ‘chlorine smell’ in swimming pools is associated with NCl3. It is highly explosive and toxic in concentrated amounts. The IUPAC name of NCl3 is trichloramine.
Many chemistry students have a query regarding the chemical nature of the nitrogen trichloride (NCl3) molecule, i.e., is it polar or non-polar?
Therefore, in this article, we will answer that question and discuss the polarity of the NCl3 molecule.
Is NCl3 polar or nonpolar?
Nitrogen trichloride (NCl3) is a weakly polar molecule. The central nitrogen (N) atom in the NCl3molecule is surrounded by three chlorine (Cl) atoms via single covalent bonds, forming an asymmetric trigonal pyramidal shape.
The electronegativity of the chlorine (Cl) atom is slightly higher than the nitrogen (N) atom. Each Cl atom strongly attracts the shared electron pair from each N-Cl bond.
Thus, each N-Cl bond in the NCl3 molecule is individually polar and possesses a specific dipole moment value.
The asymmetric shape of NCl3 further enhances the polarity effect as the N-Cl dipole moments do not get canceled in the molecule overall. Thus, NCl3 is a polar molecule with a net dipole moment value > 0.
A molecule with unequal charge distribution between different centers of bonded atoms is a polar molecule. It is formed by the covalent bonding between two different atoms leading to an asymmetric electron density.
In this case, the atoms acquire partial positive (δ+) and partial negative (δ–) charges.
If the dipole moments of individually polar bonds are not canceled due to the asymmetrical shape of the molecule, the molecule will then be polar, such as NCl3.
Hence a polar molecule has an unequal distribution of the electronic charge. Contrarily, if the electronic charge is evenly spread over the molecule, in that case, it will be a non-polar molecule overall.
The following three factors influence the polarity of any covalently bonded molecule:
Electronegativity.
Dipole moment.
Molecular geometry or shape.
In the next section, we will uncover how these factors control the polarity of the NCl3 molecule.
Factors affecting the polarity of NCl3
Electronegativity
It is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity decreases down the group in the Periodic Table of elements while it increases across a period.
Greater the electronegativity difference between bonded atoms in a molecule, the higher the bond polarity.
Nitrogen belongs to group V-A (or 15) of the Periodic Table. The electronic configuration of nitrogen is 1s2 2s2 2p3, so it has 5 valence electrons available for bonding.
On the other hand, the electronic configuration of chlorine is 1s22s22p6 3s2 3p5. Thus, each Cl-atom has 7 valence electrons as it is a halogen belonging to group VII A (or 17). Therefore each Cl atom needs 1 more electron in its valence shell to complete its octet configuration. The three Cl-atoms thus form a single covalent bond with the central N-atom on each side of the NCl3 molecule.
Three valence electrons consumed in bonding out of the five initially available leaves behind two valence electrons, i.e., one lone pair on the central N-atom in the NCl3 molecule.
Atom
Electronic configuration
Valence electrons
Nitrogen (7N)
1s22s22p3
5
Chlorine (17Cl)
1s22s22p6 3s2 3p5
7
Chlorine is slightly more electronegative than nitrogen (E.N= 3.16 vs 3.04, respectively). There is an electronegativity difference of only 0.12 units between these two atoms.
Due to this small electronegativity difference, chlorine slightly attracts the shared electron cloud from each of the three N-Cl bonds in NCl3. The bonded electrons are held significantly close to the chlorine atoms in each N-Cl bond in the NCl3 molecule.
The central N-atom thus gains a partial positive (Nδ+) charge, while the chlorine atoms having greater electronegativity obtain a partial negative (Clδ-) charge. Thus, oppositely charged poles develop in the NCl3 molecule (as shown below).
Dipole Moment
The dipole moment is the product of electrical charge (Q) and bond length (r) between two bonded atoms. It is a vector quantity expressed in Debye (D) units.
It is represented by a Greek symbol µ and measures the polarity of a bond.
It is noted that a molecule’s dipole moment depends on the electronegativity difference between the bonded atoms. The greater the electronegativity difference, the higher the bond polarity, resulting in a high dipole moment value.
It points from the partial positive (δ+) center to the partial negative (δ–) center of a bond or molecule.
The dipole moment of individually polar N-Cl bonds points from Nδ+ to Clδ- due to the electronegativity difference between the N and Cl atoms.
Molecular geometry
As discussed earlier, a nitrogen trichloride (NCl3) molecule consists of three single covalent N-Cl bonds. There are a total of 26 valence electrons in the overall molecule. The central nitrogen (N) atom carries a lone pair of electrons.
According to the Valence Shell Electron Pair Repulsion (VSEPR) theory of chemical bonding, NCl3 is an AX3E1-type molecule. Around the central nitrogen atom (A), there are three bond pairs (X) and only one lone pair of electrons (E).
A lone pair of electrons on the central N-atom leads to strong lone pair-bond pair electronic repulsions in the NCl3 molecule. These strong repulsions push the N-Cl bonds downwards and contribute to the distortion in the regular symmetry of the molecule.
The nitrogen trichloride (NCl3) molecule thus adopts a “trigonal pyramidal” shape instead of its ideal tetrahedral electron geometry. The bond angle is also reduced to 107.1° from the ideal 109.5°.
As a result of the asymmetric shape, the individual N-Cl dipole moments do not get canceled in the nitrogen trichloride (NCl3) molecule. There is an unequal electronic charge distribution over the molecule.
Thus, NCl3 is an overall polar molecule with a net dipole moment µ = 0.60 D.
Difference between polar and nonpolar?
Polar molecule
Non-polar molecule
Atoms must have a difference in
electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than zero
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3), sulfur dioxide (SO2), bromine trifluoride (BrF3), nitrogen trichloride (NCl3), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), sulfur trioxide (SO3), etc.
NCl3 has polar bonds because of a slight electronegativity difference between the bonded N and Cl atoms in each N-Cl bond.
Due to the strong lone-pair bond-pair repulsions, the NCl3 molecule adopts an asymmetric trigonal pyramidal shape. As a result, the dipole moments are not canceled in the molecule overall.
Thus, NCl3 is a polar molecule.
Why does the NCl3 molecule possess a small dipole moment value?
A small electronegativity difference of only 0.12 units exists between the bonded N and Cl atoms in each N-Cl bond.
The three Cl-atoms attract the N-Cl electron cloud, but to a small extent; thus, each N-Cl bond has a small dipole moment value. Consequently, the net dipole moment of NCl3 is small, i.e., µ = 0.60 D.
Why does an NCl3 molecule have a greater bond angle than an NH3 molecule?
A chlorine atom is bigger and more electronegative than a hydrogen atom. As a result, the Cl-Cl bond pair repulsion and the greater electronegativity of chlorine atoms lead to a slightly higher bond angle in the NCl3.
The atoms with higher electronegativity cause more repulsion. That is why lone-pair lone-pair repulsion in the NCl3 molecule is higher than in NH3, contributing to a higher bond angle.
Is there a formal charge on the bonded atoms in the NCl3 molecule?
Formal charge of an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
For chlorine atoms
Valence electrons = 7
Bonding electrons = 2
Non-bonding electrons = 6
∴ The formal charge on the chlorine atoms = 7-6-2/2 = 7-6-1 = 7-7 = 0
For nitrogen atom
Valence electrons = 5
Bonding electrons = 6
Non-bonding electrons = 2
∴ The formal charge on the central nitrogen atom = 5-2-6/2 = 5-2-3 = 5-5 = 0
Thus, no formal charge is present on the nitrogen trichloride (NCl3) molecule overall.
Summary
Nitrogen trichloride (NCl3) is a polar molecule.
It consists of three weakly polar N-Cl bonds due to an electronegativity difference of 0.12 units between the bonded atoms.
Chlorine attracts the shared electron cloud from each N-Cl bond.
The lone pair of electrons on the N-atom distort the shape and geometry of the NCl3 molecule, which enhances the polarity effect leading to an unequal charge distribution in the molecule.
Due to the unsymmetrical arrangement of atoms, the individual N-Cl dipole moments are not canceled in the molecule overall. Thus, NCl3 is polar ( net μ= 0.60 D).
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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