[NO3]– is the chemical formula for the nitrate ion. It is a nitrogen oxoanion formed by the loss of a proton from nitric acid (HNO3). It is a polyatomic ion having a molar mass of 62.01 g/mol. The nitrate ion is the most common nitrogen source for plants and vegetable crops.
To find out whether the nitrate [NO3]– ion is polar or non-polar, continue reading the article.
Is NO3– polar or nonpolar?
Nitrate [NO3]– is a non-polar molecular ion. It consists of a nitrogen (N) atom and three oxygen (O) atoms. The nitrogen atom is present at the center of the molecular ion, while three oxygen (O) atoms occupy terminal positions, one on each side, making a symmetrical trigonal planar shape.
An electronegativity difference of 0.40 units exists between a nitrogen and an oxygen atom in each of the N=O and N-O bonds in the NO3– ion. Thus, all three covalent bonds are individually polar in the NO3– molecular ion and possess a specific dipole moment value (symbol µ).
However, it is due to the symmetrical shape of NO3– that the dipole moments of individually polar N-O and N=O bonds get canceled in the ion overall. The electron cloud distribution stays uniform thus, NO3– is overall non-polar (net µ =0).
A molecule is polar if there is a non-uniform charge distribution present in it. If the charge distribution gets equally balanced in different parts, then that molecule or molecular ion is considered non-polar.
The following three factors mainly influence the polarity of a molecule or molecular ion:
The electronegativity difference between two or more covalently bonded atoms
Dipole moment
Molecular geometry or shape
Now, let us discuss the effect of the above three factors one by one to prove that the nitrate (NO3–) ion is overall non-polar.
Factors affecting the polarity of [NO3]–
Electronegativity
It is defined as the ability of an elemental atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity increases across a period in the Periodic Table while it decreases down the group.
Greater the electronegativity difference between bonded atoms in a molecule or molecular ion, the higher the bond polarity.
Nitrogen (N) is present in Group V A (or 15) of the Periodic Table. The electronic configuration of nitrogen is 1s2 2s2 2p3. As per this electronic configuration, an N-atom has a total of 5 valence electrons. It is thus short of 3 valence electrons that are required so that a nitrogen atom can achieve a complete octet electronic configuration.
Conversely, oxygen (O) belongs to Group VI A (or 16) of the Periodic Table. The electronic configuration of an oxygen atom is 1s2 2s2 2p4. So each O-atom has a deficiency of 2 more valence electrons for it to complete its octet.
Hence in NO3–, three O-atoms are covalently bonded with the central N-atom, one on each side of the trigonal planar shape. One O-atom is bonded via a double-covalent bond, while two O-atoms are single-covalently bonded to the central N-atom in NO3–. In this way, all four bonded atoms attain a complete octet configuration via chemical bonding in NO3–.
All five valence electrons of nitrogen get consumed in bond formation. Consequently, there is no lone pair of electrons on the central N-atom in NO3– to distort its symmetrical molecular geometry or shape.
Atom
Electronic configuration
Valence electrons
Nitrogen (7N)
1s22s22p3
5
Oxygen (8O)
1s2 2s2 2p4
6
Oxygen (E.N = 3.44) is more electronegative than nitrogen (E.N = 3.04). There is an electronegativity difference of 0.40 units between an N and an O-atom in each N-O bond.
The more electronegative O-atom strongly attracts the shared electron cloud away from the central N-atom in each of the N-O single and N=O double bonds present in NO3–.
Thus, the nitrogen atom present at the center of NO3– gains a partial positive (Nδ+) charge, while each terminal oxygen atom obtains a partial negative charge (Oδ-). As a result, each N=O bond is individually polar in the nitrate ion.
Dipole moment
Dipole moment (μ) is a vector quantity that points from the positive pole to the negative pole of a bond in a molecule.
It is mathematically calculated as a product of the magnitude of charge (Q) and charge separation (r).
The dipole moment is expressed in a unit called Debye (D).
The dipole moment of a polar covalent bond conventionally points from the positive center to the center of the negative charge.
So in NO3–, the dipole moment of each N=O and N-O polar bonds points from Nδ+ to Oδ-(as shown below).
Molecular geometry
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, NO3– is an AX3E0 or simply AX3-type molecular ion. To one N-atom at the center (A), two N-O and one N=O bond pairs (X) are attached, and there is no lone pair (E) on the central atom.
So, the molecular geometry or shape of NO3– is identical to its ideal electron pair geometry, i.e., trigonal planar. Each O=N-O bond angle is 120° in NO3–.
The absence of any lone pairs of electrons on the central atom in [NO3]– ion ensures neither bond pair-bond pair nor lone pair-bond pair electronic repulsion exists in the molecular ion. Thus, NO3– maintains a symmetrical shape.
It is due to this symmetrical shape that the dipole moment of an upwards-pointing N=O bond gets canceled with the net dipole moment of two downwards-pointing N-O bonds in NO3–.
The electron cloud stays uniformly distributed over the molecular ion. Thus although having polar bonds present, NO3– is overall non-polar (net µ = 0).
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), nitrite [NO2]– ion, etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), nitrate [NO3]– ion, etc.
NO3– has polar bonds present due to a specific electronegativity difference of 0.40 units between the bonded nitrogen and oxygen atoms in each N-O and N=O bond.
However, it is due to the symmetrical trigonal planar shape of NO3– that the dipole moments of individually polar N-O and N=O bonds get canceled equally in opposite directions.
Thus, NO3– is overall non-polar with a net dipole moment µ =0.
What is the electronegativity difference between the atoms of [NO3]– ion?
The electronegativity of nitrogen is 3.04, while the electronegativity of oxygen atoms in each N-O and N=O is 3.44.
Hence an electronegativity difference of 0.40 units is present between the bonded atoms in each N-O and N=O bond in [NO3]–.
What are the formal charges present on bonded atoms in NO3–?
Formal charge on an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
∴ Formal charge on each single-bonded O-atom = 6-6-2/2 = 6-6-1 = 6-7 = -1
For double-bonded O-atom
Valence electrons = 6
Bonding electrons = 4
Non-bonding electrons = 4
∴ Formal charge on the double-bonded O-atom = 6-4-4/2 = 6-4-2 = 6-6 = 0
+1 formal charge of central N-atom cancels with -1 charges on one of the two single-bonded O-atoms; thus, the overall charge present on the nitrate ion is +1+ (-1) +(-1) = -1.
Why [NO3]– ion is non-polar, but [NO2]– ion is polar?
In the nitrate [NO3]– ion, there is no lone pair of electrons on the central N-atom. The ion thus adopts a perfectly symmetrical trigonal planar shape and geometry. The dipole moments of polar N-O and N=O bonds get canceled equally. It is thus non-polar (net µ =0).
Contrarily, in the nitrite [NO2]– ion, there is a lone pair of electrons on the central N-atom. Electronic repulsions distort the shape and geometry of the molecular ion. An asymmetric bent shape of NO2– ensures the N-O and N=O dipole moments do not get canceled. Thus it is polar (net µ > 0).
Summary
Nitrate [NO3]– is a non-polar molecular ion.
It consists of polar N-O and N=O bonds due to an electronegativity difference of 0.4 units between an oxygen and a nitrogen atom.
Nitrate [NO3]– has a symmetrical trigonal planar shape with a 120° bond angle.
The electron cloud stays uniformly distributed in the symmetrical NO3– trigonal planar shape.
The dipole moment of an upwards-pointing N=O double bond gets canceled equally with the net dipole moment of two downwards-pointing N-O single bonds.
The net dipole moment in NO3– is zero, so it is overall non-polar.
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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