Dinitrogen tetroxide (N2O4) is commonly known as nitrogen tetroxide (NTO). It is colorless in solid form while in liquid and gaseous form; it has a characteristic reddish-brown color.
It has an unpleasant, irritating acid-like smell.
N2O4 exists in equilibrium with NO2 in both liquid and gaseous forms. Therefore, it is regarded as two NO2 groups that are bonded together. The reddish-brown color of the liquid form is due to the presence of the NO2 group.
Nitrogen tetroxide is a strong oxidizer. This property contributes to its high reactivity. In the air, it is non-flammable but it can inflame combustible materials.
It is highly toxic and corrosive in nature. It is used as a chemical reagent in the synthesis of various chemical compounds. When comes in contact with hydrazine fuels, N2O4 reacts spontaneously which made both of them a common rocket propellant.
The molar mass of nitrogen tetroxide is 92.011 g/mol. It’s melting and boiling points are -11.2°C and 21.69°C respectively. Its density is 1.44 g/cm3. It easily reacts with water to form nitric and nitrous acids.
Now, let us discuss other properties of N2O4 such as lewis structure, geometry, hybridization, and MO diagram.
N2O4 Lewis Structure
Lewis structure is also known as electron dot structure or Lewis dot structure. To draw these structures, the following steps should be followed-
- Determine the total number of valence electrons present in each atom of the molecule.
- Find the total number of valence electrons present in a molecule.
- Select the center atom among the atoms present in a molecule.
- Place the lone pair of electrons on atoms after drawing the molecular structure.
- Check the stability of the lewis structure and minimize the charges on the atoms.
By following these steps, the lewis structure of N2O4 can be drawn as-
Step 1– Determine the total number of valence electron present in each atom of the molecule
According to the molecular formula, there are two nitrogen atoms and four oxygen atoms are present in the N2O4 molecule.
There are 5 valence electrons present in the outermost shell of the nitrogen atom and in oxygen atoms; there are 6 valence electrons present.
Total number of valence electron in 2 nitrogen atoms- 5 * 2 = 10 electrons.
Total number of valence electron in 4 oxygen atoms- 6 * 4 = 24 electrons.
Step 2– Find the total number of valence electrons present in a molecule
Total number of valence electrons present in N2O4 molecule = No. of valence electron present in 2 nitrogen
atoms + No. of valence electron present in 4 oxygen atoms
= 10 + 24 = 34 electrons
Total number of electron pairs = Total no. of valence electrons / 2
= 34 / 2 = 17 electron pairs
= σ bonds + π bonds + lone pair of electrons
Step 3 – Select the center atom among the atoms present in a molecule
The least most electronegative atom will be selected as the center atom; therefore, in this case, the nitrogen atom is the center atom. There are two nitrogen atoms present in the N2O4 molecule, so the basic structure of N2O4 will be-
Step 4– Place the lone pair of electrons on atoms after drawing the molecular structure
According to the above structure, already there are 4 N-O bonds and 1 N-N bond present. So, 5 electron pairs (4 + 1 electron pair) are accounted for 4 N-O bonds and 1 N-N bond respectively. Now, only 12 electron pairs are remaining to draw the rest of the structure of N2O4.
Draw the remaining 12 electron pairs on oxygen atoms as lone pairs. According to the octet rule, each oxygen atom will keep 3 electron pairs.
Step 5– Place the charges on atoms
The charges should be placed in such a way that summation of charges on each atom should be equal to zero as N2O4 is a neutral molecule.
Step 6– Check the stability of the lewis structure and minimize the charges on the atoms
To draw the stable lewis structure, the charges on atoms should be minimized; therefore the above structure is not stable.
To minimize the charges, convert lone pair of electrons present on oxygen atoms to bonds. So, after converting one lone pair on one oxygen atom to N-O bond, the structure will become
Now charges are reduced but still, this new structure is unstable, so one more lone pair of electrons on another oxygen atom will be converted to N-O bond.
Now charges are reduced further and we got a stable structure. Therefore, the correct lewis structure of the N2O4 molecule is-
N2O4 Molecular Geometry
The molecular geometry of a molecule is determined by the Valence shell electron pair repulsion theory (VSEPR theory). According to the Lewis structure of N2O4, both nitrogen atoms are singly bonded to one another and singly bonded to one oxygen atom.
Also, they are doubly bonded to other oxygen atoms. Therefore, each nitrogen atom has 3 electron pairs surrounding it.
The sp2 (s + Px +Py) orbital of oxygen atoms are used to form σ N-O bonds with delocalized π bond by using unhybridized Pz orbital on each three atoms O-N-O bond.
According to VSEPR theory, the molecular geometry of the N2O4 molecule will be planar (trigonal planar on each portion of the ONO bond).
Hence, the structure of N2O4 showing its bond angle and bond length is as below.
The N-N and N-O bond lengths are 175 and 121 pm respectively. The O-N-O bond angle is 135°. The bond angle is greater than 120° because unpaired electrons repel the bonding pair less than a pair of electrons.
The hybridization of N2O4 is explained as-
Let us look at the lewis structure first; here nitrogen atom is the center atom that is attached to another nitrogen atom through a single bond. Each nitrogen atom is further attached to two oxygen atoms each. No lone pair of electrons is present on the nitrogen atom.
Therefore, each nitrogen atom is bonded to 3 electron pairs resulting in three hybridized orbitals sp2. The remaining unhybridized p-orbital will form a π bond with one of the oxygen atoms.
Hence, the hybridization of each nitrogen atom is sp2.
N2O4 Molecular Orbital (MO) Diagram
A molecular orbital (MO) diagram explains the chemical bonding in molecules by energy level diagrams. They were proposed by Robert S. Mulliken and Friedrich Hund in 1928.
As we know N2O4 molecule is a dimer of the NO2 molecule, hence we’ll discuss molecular orbital diagram of NO2 molecule first.
The electronic configuration of nitrogen is 1s2 2s2 2p3 whereas that of oxygen is 1s2 2s2 2p4. The 1s2 orbital of nitrogen and oxygen atom will take part in bonding; therefore, only valence orbitals are taken into consideration.
Thus, the NO2 molecule contains 17 valence electrons in its outermost shell. However, the 2s orbital of the oxygen atom is very low in energy so it also does not take part in bonding. The MO electronic configuration of NO2 molecule is KK [σ2s2, σ2pz2, π2px2, π2py2, πn2px2, πn2py2, π*2px1].
It has one unpaired electron in π*2px1 orbital of NO2 molecule, hence it is paramagnetic in nature. Whereas, on dimerization, this unpaired electron will get paired up forming an N2O4 molecule.
Hence, nitrogen tetroxide is diamagnetic in nature.
Polarity of N2O4
In chemistry, polarity refers to an electric charge separation resulting in the electric dipole moment of a molecule with positive and negative end dipoles.
Polar molecules are those molecules having an electronegativity difference between the atoms. On the other hand, non-polar molecules are those where atoms shared an equal number of electrons or where polarity will be canceled out in a molecule.
In the NO2 molecule, the oxygen atom is more electronegative than the nitrogen atom, hence the oxygen atom pulls electrons towards itself. Therefore, the N-O bond is polar in nature.
On dimerization, the N-N bond of N2O4 is non-polar in nature and the N-O bond is polar in nature. But, due to symmetry in a molecule, the dipole moments around N-O bonds will cancel out each other resulting in non- polar nature of the molecule.
You can also read out the article written on the polarity of NO2.
So, nitrogen tetroxide is a non-polar molecule.
In this article, we have discussed Lewis structure, molecular geometry, hybridization, Molecular orbital diagram, and polarity of nitrogen tetroxide molecule.
After this, one can easily go through any other property related to nitrogen tetroxide. This article covers all the basic properties.