Which of ${\mathrm{Ne}}_2, {\mathrm{O}}_2, {\mathrm{C}}_2$ and ${\mathrm{Li}}_2$ has/have double bond?

${\mathrm{Ne}}_2$ has B.O. zero, ${\mathrm{Li}}_2$ has B.O. 1 while ${\mathrm{O}}_2$ and ${\mathrm{C}}_2$ both have B.O  is 2.

${\mathrm{O}}_2(\mathrm{Z} = 8) : {(\mathrm{\sigma } 1\mathrm{s})}^2, {(\mathrm{\sigma }*1\mathrm{s})}^2, {(\mathrm{\sigma } 2\mathrm{s})}^2, {(\mathrm{\sigma }*2\mathrm{s})}^2, {(\mathrm{\sigma } 2{\mathrm{p}}_{\mathrm{z}})}^2$${\left(\mathrm{\pi }2{\mathrm{P}}_{\mathrm{x}}\right)}^2 ={\left(\mathrm{\pi }2{\mathrm{P}}_{\mathrm{y}}\right)}^2 ,{\left({\mathrm{\pi }}^{*}2{\mathrm{P}}_{\mathrm{x}}\right)}^1={\left({\mathrm{\pi }}^{*}2{\mathrm{P}}_{\mathrm{y}}\right)}^1$

$$\begin{gathered} \mathrm{B}.\mathrm{O}=\frac{1}{2}\left(10-6\right)=2 \\ {\mathrm{C}}_2\left(\mathrm{Z}=6\right):{ \left(\mathrm{\sigma }1\mathrm{s}\right)}^2,{ \left(\mathrm{\sigma }*1\mathrm{s}\right)}^2,{ \left(\mathrm{\sigma }2\mathrm{s}\right)}^2,{ \left(\mathrm{\sigma }*2\mathrm{s}\right)}^2,{ \left(\mathrm{\pi }2{\mathrm{p}}_{\mathrm{x}}\right)}^2={\left(\mathrm{\pi }2{\mathrm{p}}_{\mathrm{y}}\right)}^2 \\ \mathrm{B}.\mathrm{O}=\frac{1}{2}\left(8-4\right)=2 \end{gathered}$$