`._(92)^(238)U` has 92 protons and `238` nucleons. It decays by emitting an alpha particle and becomes:

Options

A. `._(92)^(234)U`
B. `._(90)^(234)U`
C. `._(92)^(235)Th`
D. `._(93)^(237)Th`

Answer

To solve the problem of what element `._(92)^(238)U` decays into after emitting an alpha particle, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Initial Element:**
The element given is Uranium-238, denoted as `._(92)^(238)U`. Here, the atomic number (number of protons) is 92, and the mass number (total nucleons) is 238.

2. **Understand Alpha Decay:**
An alpha particle is essentially a helium nucleus, which consists of 2 protons and 2 neutrons. Therefore, when an alpha particle is emitted, the atomic number decreases by 2, and the mass number decreases by 4.

3. **Calculate the New Atomic Number:**
After the emission of an alpha particle:
- New atomic number = Original atomic number - 2
- New atomic number = 92 - 2 = 90

4. **Calculate the New Mass Number:**
After the emission of an alpha particle:
- New mass number = Original mass number - 4
- New mass number = 238 - 4 = 234

5. **Identify the New Element:**
The new element formed will have an atomic number of 90 and a mass number of 234. The element with atomic number 90 is Thorium (Th).

6. **Write the Final Result:**
Therefore, the decay of Uranium-238 by emitting an alpha particle results in Thorium-234, which can be denoted as `._(90)^(234)Th`.

### Final Answer:
`._(90)^(234)Th` (Thorium-234)

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