Iodine azide, IN $_{3}$, adds to alkenes by an electrophilic mechanism similar to that of bromine. If a monosubstituted alkene such as 1 -butene is used, only one product results:
$$\mathrm{CH}_{3} \mathrm{cH}_{2} \mathrm{CH}=\mathrm{CH}_{2}+\mathrm{I}-\mathrm{N}=\mathrm{N}=\mathrm{N} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHCH}_{2} \mathrm{I}$$
(a) Add lone-pair electrons to the structure shown for IN $_{3}$, and draw a second resonance form for the molecule.
(b) Calculate formal charges for the atoms in both resonance structures you drew for IN $_{3}$ in part (a).
(c) In light of the result observed when IN $_{3}$ adds to 1 -butene, what is the polarity of the $\mathrm{I}-\mathrm{N}_{3}$ bond? Propose a mechanism for the reaction using curved arrows to show the electron flow in each step.