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NO$_2$ Molecular Geometry

In chemistry we make use of valence shell electronic pair repulsion theory in order to give or predict the shapes of every possible molecules we have around us. The idea of VSEPR theory is very much necessary in order to describe the bonding of any molecule as well as detailing of chemical properties and possible reactions of the molecules.

The entire molecular reactivity series as well the set of properties depend upon how the overall molecular structure shows with and the structure or the geometry of molecules. Nitrogen dioxide is no exception and the molecular geometry is very important to understand the nature, and chemical characteristics of the molecule.

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NO$_2$ Molecular Geometry Drawing

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The valence shell electronic pair repulsion involves only covalent bonding in molecules which shares electron pairs between the electrons of the participating atoms. When the electrons are assigned to the participating atoms bonding as well as non-bonding electron pairs we can give us an insight into the possible spatial bonds arrangement by just simple arrangement and spacing of electrons as far as possible because the proximity of electrons causes bending.

The Nitrogen dioxide molecule drawing needs to be understood from the Lewis structure first. Like every molecules and ions where more than one satisfactory lewis structure can be drawn but the octet rule has to be satisfied. Neither of the structures can satisfy nor can be considered as correct as the alternate presence of double and single bond between Nitrogen and Oxygen are distinguishable.

The reason the structures are never considered as stable or satisfactory because the double bond has more density of electrons as well as the length of the bond is shorter than single bonds. Due to these anomalies the structure of nitrogen di oxide molecule has no single stable structure but more than one resonance structures which satisfy the partial stability these individual structures show. 
NO2 Molecular Geometry

This structure implies that NO2 is a symmetrical having partial double bond character in each of the nitrogen oxygen bonds. The tracking of electrons in such a structure requires specific special notation and we need to write more than one lewis structure which are resonating to each other and connect them with a symbol implying that superimposing these structures and finally arrive at a reasonable representation of the molecule.

NO$_2^{-}$ Lewis Structure Molecular Geometry

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The molecular shape or basic geometry is determined by the electron group geometry and the overall electron cloud along with the ligand number. NO2- is a symmetrical ion having partial double bond character in each of the nitrogen oxygen bonds. 

NO2 Lewis Structure Molecular Geometry

The six electrons in valence shell of oxygen, along with five valence shell electrons of Nitrogen are the ones which are responsible for the structure that we get to see at the end. The defect of oxygen atoms sharing two pairs of electrons not taking into account the geometry that is involved. There is a presence of double bond in between two oxygen atoms when oxygen molecule is formed. A triple bond must be assumed in case of N2 to give each nitrogen atom a noble gas configuration. When these atoms combine together to form the nitrite ion $NO_{2}^{-}$ the alternate presence of double bond between nitrogen and oxygen on one side and single bond between nitrogen and oxygen on the arm.

As these bonds in between nitrogen and oxygen is not that stable we create two resonating structures instead of one structure and hence both of these nitrite ion structures suffix each other as well as compensate the absence of stability in one structure.
  • In order to understand these better we refer VSEPR as the lewis structures help us find the steric number of the given molecule’s central atom or atoms.
  • The steric number provides the hybridization as well as electron group geometry. The number of ligands basically is the number of atoms which are bonded to central atom.
  • Once the electron pair repulsion is visualised or described the theory helps us to draw geometric representation of each which we designate as orbital overlap pattern.
  • The sketch helps in displaying nitrogen atom represented with a letter or text and are surrounded by cloud of electrons including the lone pair as well as bonded pairs.
  • The bonded pairs of electrons can either be sigma (σ) or pi (π) bonds.
  • Each of these help in getting the right orbital overlap sketch which in turn help in getting the exact bend and overall angle of the molecule, in this case nitrite $NO_{2}^{-}$ ion.

NO$_2^{-}$ Molecular Geometry Bond Angle

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In nitrite ion, N atom is $sp_{2}$ hybridised with the presence of one lone pair of electron and hence the bond angle is 115o and 120 degree. The bond angles of $NO_{2}$, $NO_{2}^{+}$ and $NO_{2}^{-}$ are in the order $NO_{2}^{+} > NO2 > NO_{2}^{-}$.
This is mainly because $NO_{2}^{+}$ has no unshared electron and it is linear. $NO_{2}$ has one unshared electron while $NO_{2}^{-}$ has one unshared electron pair. 

NO2 Molecular Geometry Bond Angle
The consideration of NO2- and many other molecules and ions shows that simple layout for overall number of electrons and assisting them to the atoms valence shells with either bonded or unshared pairs is not entirely satisfactory.

Fortunately the simple model may be altered easily to fit in many of the awkward cases. The problem with $NO_{2}^{-}$ is that the ion is actually more symmetrical than either one of the lewis electronic structures that was observed before. The two structures when superimposed show a new structure having same symmetry as the molecule. These structures also imply that NO2- is a hybrid of two resonance structures and when two or more resonance structures are drawn for a molecule or ion the electronic formula for the species is considered to be a resonance hybrid structures.

NO$_2^{-}$ Ion Molecular Geometry

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To understand better the nitrite ion molecular geometry we need to understand the NO2 structure better. The trigonal planar structure of $NO_{2}^{-}$ has lot to do with the main $NO_{2}$ structure geometry. $NO_{2}$ structure is usually considered as unusual and although its bond length of 119.7 pm lies exactly in between $NO_{2}^{-}$ of 123.6 pm and NO2+ of 115 pm, its bond angle lies outside the normal ranges of bond angles.

$NO_{2}$ has 17 valence shell electrons and these are split into 8 of one spin, and 9 of the other. The 9 member spin set has 3 electrons in bonding regions which are same as the number of bonding electron pairs in 9 electron pair. The unshared electron pairs on the central N atom explains the overall the molecular geometry of all three forms of $NO_{2}$.

In $NO_{2}^{+}$, the N- atom has no unshared electrons and so the molecule is linear having bond angle of 180 degree. In NO2, the N atom has one unshared electron causing less repulsion than in $NO_{2}^{-}$ in which the N atom has unshared electron pair causing more repulsion. This results the bond pairs in $NO_{2}^{-}$ being more close than $NO_{2}$.

The $NO_{2}’s$ 134 degree bond angle is not optimal for either its 8 or 9 member spin set. Both spin sets of $NO_{2}$ are strained and this strain energy for the 9 member spin set on its departure from its optimal bent geometry rises faster than the strain energy of 8 member spin on its departure from its optimal linear geometry and not only due to the 9 member spin set having greater number of electrons.

The total bond order is found to be $\left(\frac{1}{2}\right )$ $\frac{( 3 + 4)}{2}$ = 1.75 which matches the bond order inferred from bond lengths, which is average of 1.5 of $NO_{2}^{-}$ and 2 of $NO_{2}^{+}$. 

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