Assertion (A) : The oxygen - oxygen bonds in the ${\text{O}}_3$molecule are intermediate between a single and double bond .

Reason (R) : Bond order of  ${\text{O}}_3$ is 1.5 

The resonance structures of ozone (${\text{O}}_3$) molecule are shown below:

Bond order of ozone (${\text{O}}_3$) molecule 

 $\text{=\hspace{0.17em}}\frac{\text{Total\hspace{0.17em}number\hspace{0.17em}of\hspace{0.17em}bonds\hspace{0.17em}between\hspace{0.17em}two\hspace{0.17em}atoms\hspace{0.17em}in\hspace{0.17em}all\hspace{0.17em}resonating\hspace{0.17em}structures}}{\text{Total\hspace{0.17em}number\hspace{0.17em}of\hspace{0.17em}resonating\hspace{0.17em}structures}}$

$=\frac{2+1}{2}=\frac{3}{2}=1.5$

The oxygen - oxygen bonds in the ${\text{O}}_3$molecule are intermediate between a single and double bonds, because bond order of  ${\text{O}}_3$ is 1.5.