Duplet configuration is not found in

Options

A. Hydride ion
B. Hydrogen molecule
C. Lithium cation
D. `Be^(3+)`

Explanation

To determine where the duplet configuration is not found, we first need to understand what duplet configuration means. The duplet configuration refers to the electron arrangement of atoms that have two electrons in their outermost shell, similar to the noble gas helium, which has a configuration of 1s².

### Step-by-Step Solution:

1. **Understanding Duplet Configuration**:
- The duplet configuration is characterized by having two electrons in the outermost shell. The most common example is helium, which has the electron configuration of 1s².

2. **Analyzing Different Species**:
- **Hydride Ion (H⁻)**:
- The electron configuration for hydrogen (H) is 1s¹. When it gains an electron to form H⁻, the configuration becomes 1s², which follows the duplet rule.
- **Hydrogen Molecule (H₂)**:
- In H₂, two hydrogen atoms share their electrons. Each hydrogen atom contributes one electron, resulting in a total of two electrons (1s² configuration), thus following the duplet rule.
- **Lithium Cation (Li⁺)**:
- Lithium has an atomic number of 3, with the electron configuration of 1s² 2s¹. When it loses one electron to become Li⁺, the configuration becomes 1s², which also follows the duplet rule.
- **Beryllium Cation (Be³⁺)**:
- Beryllium has an atomic number of 4, with the electron configuration of 1s² 2s². When it loses three electrons to form Be³⁺, it loses both 2s electrons and one 1s electron, resulting in a configuration of 1s¹. This does not satisfy the duplet configuration, as it has only one electron in the outer shell.

3. **Conclusion**:
- Among the species analyzed, the beryllium cation (Be³⁺) does not exhibit a duplet configuration because it has only one electron (1s¹) instead of two.

### Final Answer:
Duplet configuration is not found in **Beryllium cation (Be³⁺)**.