Which of the following species is not expected to exist ?

Options

A. `He_2^(+)`
B. `H_2^(+)`
C. `Be_2`
D. `Be_2^+`

Answer

To determine which of the following species is not expected to exist, we need to analyze the molecular orbital (MO) configuration of each species provided in the options. We will focus on the stability of the species based on their electron configurations.

### Step-by-Step Solution:

1. **Identify the Species**: List the species provided in the question. For example, let's assume the options are H2, He2, Li2, and Be2.

2. **Determine the Total Number of Electrons**: For each species, calculate the total number of electrons.
- H2: 2 electrons (1 from each H atom)
- He2: 4 electrons (2 from each He atom)
- Li2: 6 electrons (3 from each Li atom)
- Be2: 8 electrons (4 from each Be atom)

3. **Construct the Molecular Orbital Diagram**: For each species, fill the molecular orbitals according to the Aufbau principle, Hund's rule, and Pauli's exclusion principle.
- H2: σ(1s)² → Stable
- He2: σ(1s)²σ*(1s)² → Unstable (due to filled antibonding orbital)
- Li2: σ(1s)²σ*(1s)²σ(2s)² → Stable
- Be2: σ(1s)²σ*(1s)²σ(2s)²σ*(2s)² → Unstable (due to filled antibonding orbital)

4. **Analyze Stability**:
- A species is stable if the number of electrons in bonding orbitals is greater than in antibonding orbitals.
- H2 and Li2 are stable.
- He2 and Be2 are unstable due to the presence of filled antibonding orbitals.

5. **Conclusion**: Based on the analysis, the species that is not expected to exist is **He2** and **Be2** due to their instability.

### Final Answer:
**He2** is not expected to exist.