How many among the given species have a bond order of 0 . 5 ? H 2 + , He 2 + , He 2 − , B 2 + , F 2…

As according to the concept of molecular orbital theory the bond order depends on number of bonding electrons and anti bonding electrons and can be given by with the formula:

$\mathrm{BO} =\frac{ \mathrm{bonding} \mathrm{electrons}-\mathrm{antibonding} \mathrm{electrons}}{2}$

Now, according to the digram of molecular orbital theory there are four molecules which show bond order is equal to $0.5$ these are

${{\mathrm{H}}_2}^{+}, {{\mathrm{He}}_2}^{+}, {{\mathrm{He}}_2}^{-}$ and ${{\mathrm{B}}_2}^{+}$