Q10.60P - Chemistry: Molecular Nature Of Matter And Change

Question

Dinitrogen difluoride, ${\mathbf{N}}_2{\mathbf{F}}_2$, is the only stable, simple inorganic molecule with an N=N bond. The compound occurs in cis and trans forms.(a) Draw the molecular shapes of the two forms of ${\mathbf{N}}_2{\mathbf{F}}_2$.(b) Predict the polarity, if any, of each form.

Step-by-Step Solution

Answer

(a) The molecular shapes of the two forms of ${\mathbf{N}}_2{\mathbf{F}}_2$ are:

(b) The cis form of dinitrogen difluoride is polar while the transform is nonpolar.

Step 1: Definition of Molecular Polarity, Dipole Moment, Geometrical Isomers

Molecules that share a net imbalance of charge show molecular polarity. The product of the partial charges and the distance between them is called the dipole moment $\mathbf{\mu }$.

Geometrical isomers are those compounds that have the same molecular formula but different structural arrangements. 

• A bond is said to be polar when it joins the atoms with different electronegativity. 
• In diatomic molecules with only two atoms with one bond, the molecule is polar due to bond polarity. 
• In molecules with more than two atoms, shape and bond polarity will give the molecular polarity. 
• Polar molecules have partial charges around the atoms showing dipole moment.
• The polarity of a bond is determined by the atom’s electronegativity values. 
• We also know that electronegativity values of elements increase across the period and decrease down the group. 
• Calculating the electronegativity difference  $\left(\mathrm{\Delta EN}\right)$ of the molecules will help to find out which molecule is more polar. 
• Molecules with a high value of $\mathbf{\left(}\mathbf{\Delta EN}\mathbf{\right)}$ has the most polar bond with a greater dipole moment.

Step 2: Molecular Shapes of N 2 F 2

• The stability of dinitrogen difluoride ${\mathbf{N}}_2{\mathbf{F}}_2$ is due to its structural arrangement to avoid interelectronic repulsion. It has two forms of structure. They are geometrical isomers. 
• The form in which both the fluorine atoms are arranged in the opposite direction attached to N=N is called the trans-form.
• The form in which both the fluorine atoms are arranged on the same side to  is called the cis-form.
• The lone pairs on the N atom play an important role in its structure. Due to the lone pairs on N, rather than arranging in a straight line,${\mathbf{N}}_2{\mathbf{F}}_2$ takes a trigonal planar shape

The molecular shapes of the two forms of ${\mathbf{N}}_2{\mathbf{F}}_2$ are:

 

Step 3: Polarity of two forms of N 2 F 2

Let us find whether the molecule ${\mathbf{N}}_2{\mathbf{F}}_2$ has polarity or not. 

The molecule ${\mathbf{N}}_2{\mathbf{F}}_2$ has a trigonal planar shape. As $\mathbf{F}\left(\mathbf{EN}\mathbf{=}\mathbf{4}\mathbf{.}\mathbf{0}\right)$ has more electronegativity value than $\mathbf{N}\left(\mathbf{EN}\mathbf{=}\mathbf{3}\mathbf{.}\mathbf{04}\right)$ with an electronegativity difference of 0.96, making the N-F bond more polar as each bond dipole points towards F. It has two geometrical isomers. The lone pairs on N also influence the bond polarity.

The trans-form of ${\mathbf{N}}_2{\mathbf{F}}_2$ has no dipole moment as the N-F bond polarities are counterbalanced as both the fluorine atoms are arranged in the opposite direction. Thus, the molecule does not show a significant dipole moment making the molecule nonpolar. 

While, the cis-form of ${\mathbf{N}}_2{\mathbf{F}}_2$ has a significant dipole moment as both N-F bonds polarities reinforce each other and are pointed towards F making the bond polarity not to be counterbalanced as both the fluorine atoms are arranged on the same side. Thus, the molecule shows a significant dipole moment making the molecule polar.