Bohr's theory is applicable to

Options

A. `Li^(+2)`
B. `Li^(+)`
C. `He^(+)`
D. Both 1 and 3

Answer

To determine the applicability of Bohr's theory, we need to analyze the options provided in the question based on the number of electrons in each species.

### Step-by-Step Solution:

1. **Understanding Bohr's Theory**:
Bohr's theory is primarily applicable to hydrogen-like (mono-electronic) species, which means it can only be used for atoms or ions that have only one electron.

2. **Analyzing the Options**:
- **Option 1: Lithium ion (Li²⁺)**
- Lithium (Li) has an atomic number of 3, meaning it has 3 electrons.
- The ion Li²⁺ means it has lost 2 electrons, leaving it with 1 electron.
- Since it has only one electron, Bohr's theory is applicable to Li²⁺.

- **Option 2: Lithium ion (Li⁺)**
- The Li⁺ ion has lost 1 electron, leaving it with 2 electrons.
- Since it has more than one electron, Bohr's theory is not applicable to Li⁺.

- **Option 3: Helium ion (He⁺)**
- Helium (He) has an atomic number of 2, meaning it has 2 electrons.
- The He⁺ ion means it has lost 1 electron, leaving it with 1 electron.
- Since it has only one electron, Bohr's theory is applicable to He⁺.

3. **Conclusion**:
- From the analysis, we find that Bohr's theory is applicable to both Li²⁺ (Option 1) and He⁺ (Option 3).
- Therefore, the correct answer is **Option 4: Both 1 and 3**.

### Final Answer:
Bohr's theory is applicable to both Li²⁺ and He⁺, so the correct answer is **Option 4: Both 1 and 3**.