According to Molecular Orbital Theory (MOT), electronic configuration and their bond order of given options are as follows - O22−=σ1s2,σ*1s2,σ2s2,σ*2s2,σ2pz2,π2px2,π2py2,π*2px2,π*2py2 [No. of bonding electrons Bond order =r=
− No. of antibonding electrons]
2
=
10−8
2
=
2
2
=1 - He2−=σ1s2,σ*1s2,σ2s1 Bond order =
3−2
2
=
1
2
=0.5 - Be2=σ1s2,σ*1s2,σ2s2,σ*2s2 Bond order =
4−4
2
=0 - He2+=σ1s2,σ*1s1 Bond order =
2−1
2
=
1
2
=0.5 If bond order of chemical species is zero then that chemical species does not exist. Therefore, Be2 does not exist.