Bohr's theory is not applicable to-

Options

A. He
B. `Li^(2+)`
C. `He^(2+)`
D. the H-atom

Answer

To determine where Bohr's theory is not applicable, we need to understand the limitations of Bohr's model of the atom. Here’s a step-by-step solution:

### Step 1: Understand Bohr's Theory
Bohr's theory primarily applies to hydrogen-like atoms, which are atoms that have only one electron. This model was developed to explain the spectral lines of hydrogen.

**Hint:** Recall that Bohr's model is designed for single-electron systems.

### Step 2: Identify Hydrogen-like Atoms
Hydrogen-like atoms include hydrogen (H) and ions like lithium (Li²⁺), which have only one electron.

**Hint:** Look for atoms or ions that have a single electron.

### Step 3: Analyze the Given Options
- **Hydrogen (H)**: Has one electron, so it follows Bohr's theory.
- **Lithium (Li²⁺)**: Also has one electron after losing two electrons, so it follows Bohr's theory.
- **Helium (He)**: Has two electrons, so it does not follow Bohr's theory.
- **Helium ion (He⁺)**: Has one electron, so it follows Bohr's theory.

**Hint:** Count the number of electrons in each atom or ion to determine applicability.

### Step 4: Conclusion
Bohr's theory is not applicable to helium (He) because it has two electrons. Thus, the correct answer is that Bohr's theory is not applicable to helium.

**Final Answer:** Bohr's theory is not applicable to helium (He).