A molecule/ion among the following species that does not exist under normal conditions is:

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1. **Criterion for Existence:** According to Molecular Orbital Theory (MOT), a molecule is viable only if its **Bond Order** is positive (greater than zero).
2. **Bond Order Formula:** $ 	ext{Bond Order} = \frac{1}{2}(N_b - N_a) $, where $N_b$ is the number of bonding electrons and $N_a$ is the number of antibonding electrons.
3. **Analyze Be₂:**
   - Beryllium (Be, Z=4) has 4 electrons. The molecule $Be_2$ has a total of 8 electrons.
   - Configuration: $\sigma_{1s}^2 \sigma_{1s}^{*2} \sigma_{2s}^2 \sigma_{2s}^{*2}$.
   - $N_b = 4$ ($2$ in $\sigma_{1s}$, $2$ in $\sigma_{2s}$).
   - $N_a = 4$ ($2$ in $\sigma_{1s}^*$, $2$ in $\sigma_{2s}^*$).
   - Calculation: $	ext{Bond Order} = \frac{1}{2}(4 - 4) = 0$.
4. **Conclusion:** A bond order of zero implies no net bond forms between the atoms. Therefore, **$Be_2$ does not exist**.

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