Which of the following statements about the molecule Be_2 is correct?

• A. Be_2 is not stable under normal conditions because its bond order is zero.
• B. Be_2 is stable and commonly found in nature.
• C. Be_2 is stable because it has a bond order of two.
• D. Be_2 is stable only at very high temperatures.

Answer: (A)

Verified step by step guidance

1. Recall that the stability of a diatomic molecule like Be_2 can be analyzed using Molecular Orbital (MO) theory, which involves calculating the bond order from the molecular orbital configuration.
2. Write the electron configuration for a single Be atom: 1s^2 2s^2. Since Be has 4 electrons, Be_2 will have a total of 8 electrons to place in molecular orbitals.
3. Fill the molecular orbitals for Be_2 in order of increasing energy: the 1s orbitals combine to form bonding and antibonding orbitals (usually ignored for bonding considerations), then the 2s orbitals combine to form a bonding (\( \sigma_{2s} \)) and antibonding (\( \sigma_{2s}^* \)) orbital.
4. Count the electrons in bonding and antibonding orbitals: place 4 electrons in the bonding \( \sigma_{2s} \) orbital and 4 electrons in the antibonding \( \sigma_{2s}^* \) orbital.
5. Calculate the bond order using the formula: \[ \text{Bond order} = \frac{(\text{number of electrons in bonding orbitals}) - (\text{number of electrons in antibonding orbitals})}{2} \]. For Be_2, this results in zero, indicating no net bond and thus instability under normal conditions.