Q10.68CP - Chemistry: Molecular Nature Of Matter And Change

Question

The actual bond angle in ${\mathbf{NO}}_{\mathbf{2}}$ is $\mathbf{134}.{\mathbf{3}}^o$, and in ${\mathbf{NO}}_{\mathbf{2}}^{\mathbf{-}}$ it is $\mathbf{115}{\mathbf{.4}}^o$, although the ideal bond angle is ${\mathbf{120}}^o$ in both. Explain.

Step-by-Step Solution

Answer

The actual bond angle in ${\mathrm{NO}}_2$ is $134.3^o$, and in ${\mathrm{NO}}_2^{-}$ it is $115.4^o$, although the ideal bond angle is $120°$ in both, this is due to the presence of lone electron in ${\mathrm{NO}}_2$ and lone pair of electrons ${\mathrm{NO}}_2^{-}$.

Step 1: Definition of Lewis Structure

Lewis structure is the two dimensional structure which contains electron dot symbols of every atom present and the bonding pairs that hold them together. 

The molecular shapes of the molecules from Lewis structure are determined by employing Valence-Shell Electron-Pair Repulsion (VSEPR) theory. The molecular shapes are assigned specific ${\mathrm{AX}}_{\mathrm{m}}{\mathrm{E}}_{\mathrm{n}}$ designations.

Where m and n are integers

A - Central atom

X - Surrounding atom 

E - Nonbonding valence-electron group (usually a lone pair).

Step 2: Bond Angles of and NO 2 − Molecule

The ${\mathrm{NO}}_2$ molecule is an odd electron molecule, that is the number of valence electrons in ${\mathrm{NO}}_2$ molecule is not even.

It contains total of 17 electrons. 5 electrons from N and 12electrons from two O. As per VSEPR theory, due to three electron domains ${\mathrm{NO}}_2$ molecule is having trigonal planar arrangement for which the ideal bond angle is ${120}^o$.  

But due to lone pair of electrons in ${\mathrm{NO}}_2$, the molecular is considered to have designation ${\mathrm{AX}}_2{\mathrm{E}}_{0.5}$, with bent shape. It also has resonance structures. As the single electron repulsive power is less, the N-O-N bond increases, making the bond angle $134.3^o$ and not $120°$.

Whereas the ${\mathrm{NO}}_2^{-}$ molecule is not an odd electron molecule.

It contains total of 18 electrons. 5 electrons from N and 12 electrons from two O and 1 electron from the negative charge. As per VSEPR theory, due to three electron domains ${\mathrm{NO}}_2^{-}$ molecule is also having trigonal planar arrangement for which the ideal bond angle is ${120}^o$.  

But due to lone pair of electrons in ${\mathrm{NO}}_2^{-}$,the molecular is considered to have designation ${\mathrm{AX}}_2\mathrm{E}$, with bent shape. It also has resonance structures. The presence of lone pair exerts greater repulsion than the bonding pairs decreasing the N-O-N bond angle. Thus, N-O-N bond angle in ${\mathrm{NO}}_2^{-}$ will be $115.4^o$ and not ${120}^o$.

Thus, due to the presence of lone electron in ${\mathrm{NO}}_2$ and lone pair of electrons in ${\mathrm{NO}}_2^{-}$, the actual bond angle in ${\mathrm{NO}}_2$ and ${\mathrm{NO}}_2^{-}$ is not ${120}^o$, which is the ideal bond angle.