For emission of `alpha`-particle from uranium nucleus:
`._(92)U^(235) - ._(2)He^(4) rarr ._(90)Th^(231)`
Shortage of two electrons in thorium is due to

Options

A. Conversion of electron to positron
B. Adsorption in the nucleus
C. Annihilation
D. Combustion with position to evolve energy

Answer

To solve the question regarding the shortage of two electrons in thorium after the emission of an alpha particle from uranium, we can break down the process step by step.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
The given nuclear reaction is:
\[ _{92}^{235}U \rightarrow _{2}^{4}He + _{90}^{231}Th \]
Here, uranium-235 emits an alpha particle (which is a helium nucleus, \(_{2}^{4}He\)) and transforms into thorium-231.

2. **Analyzing Atomic Numbers**:
- The atomic number of uranium (U) is 92.
- The atomic number of the emitted alpha particle (He) is 2.
- The atomic number of thorium (Th) is 90.

When uranium emits an alpha particle, it loses 2 protons (which is the atomic number of the alpha particle). Therefore:
\[ 92 - 2 = 90 \]
This confirms that the atomic number of thorium is indeed 90.

3. **Electrons and Atomic Charge**:
In a neutral atom, the number of electrons equals the number of protons. Since thorium has an atomic number of 90, it has 90 protons. Therefore, in its neutral state, thorium would also have 90 electrons.

4. **Effect of Alpha Emission**:
When uranium emits an alpha particle, it loses 2 protons and 2 neutrons. This process does not directly involve the loss of electrons. However, the formation of thorium from uranium means that there is a change in the nuclear structure, which can affect the electron configuration.

5. **Shortage of Electrons**:
The shortage of 2 electrons in thorium can be attributed to the fact that the thorium nucleus is formed from a nucleus that had more protons (and thus electrons) before the emission of the alpha particle. The emission of the alpha particle leads to a change in the atomic structure, which can result in the loss of electrons due to the instability of the new nucleus.

6. **Conclusion**:
The shortage of two electrons in thorium after the emission of the alpha particle can be attributed to the nuclear reaction that alters the atomic structure, leading to a need for fewer electrons to maintain neutrality.

### Final Answer:
The shortage of two electrons in thorium is due to the emission of an alpha particle from uranium, resulting in a change in the atomic structure and a reduction in the number of electrons needed for electrical neutrality.

---