Which species has a bond order of 1 and does not exist as a stable molecule?
For Be₂ : (sigma 1s)² (sigma^* 1s)² (sigma 2s)² (sigma^* 2s)² . Bonding = 4, antibonding = 4. Bond order = (4 - 4/2) = 0 , but Li₂ (bond order 1) is stable. Adjusting for intent: Be₂ is typically unstable despite theoretical 1 in some excited states; He₂⁺ fits better with bond order 0.5. Correcting: H₂⁻ has bond order 0.5, unstable in neutral context.
Ref: NCERT Class 11 Chemistry > Chapter 4: Chemical Bonding and Molecular Structure > Topic: Hydrogen Bonding and Resonance and Dipole Moment