Bohr's theory is applicable to

Options

A. H atom
B. He ion
C. `LI^(+)` Ions
D. All of these

Answer

To determine the applicability of Bohr's theory, we need to analyze the options provided in the question. Bohr's theory primarily applies to hydrogen and hydrogen-like atoms, which are atoms that have only one electron.

### Step-by-Step Solution:

1. **Understanding Bohr's Theory**:
- Bohr's theory was developed to explain the spectral lines of hydrogen. It is applicable to atoms that have only one electron or can be treated as hydrogen-like.

2. **Analyzing the Options**:
- **Option A: Hydrogen Atom**:
- Hydrogen (H) has one electron. Therefore, Bohr's theory is applicable.
- **Option B: Helium Atom**:
- Helium (He) has two electrons. Since it has more than one electron, Bohr's theory is not applicable.
- **Option C: Lithium Ion (Li⁺)**:
- Lithium (Li) has three electrons. However, if it loses one electron to form Li⁺, it has two electrons left. Thus, Bohr's theory is not applicable.
- **Option D: All of These**:
- Since Bohr's theory does not apply to helium and lithium ion, this option is incorrect.

3. **Conclusion**:
- The only atom among the options where Bohr's theory is applicable is the hydrogen atom (Option A).

### Final Answer:
Bohr's theory is applicable to **Option A: Hydrogen atom**.