The mass number of the element obtained when tritium undergoes `beta` - decay is `"__"`

Answer

To find the mass number of the element obtained when tritium undergoes beta decay, we can follow these steps:

### Step-by-Step Solution:

1. **Identify Tritium**:
Tritium is an isotope of hydrogen with a mass number of 3 and an atomic number of 1. Its notation is written as \( ^3_1H \).

2. **Understand Beta Decay**:
In beta decay, a neutron in the nucleus is converted into a proton, and a beta particle (which is an electron) is emitted. The reaction can be represented as:
\[ n \rightarrow p + e^- \]
where \( n \) is a neutron, \( p \) is a proton, and \( e^- \) is the emitted beta particle.

3. **Determine Changes in Atomic and Mass Numbers**:
- The atomic number increases by 1 because a neutron is converted into a proton.
- The mass number remains unchanged because the total number of nucleons (protons + neutrons) does not change.

4. **Calculate the New Element**:
After beta decay, the new element will have:
- Atomic number: \( 1 + 1 = 2 \)
- Mass number: \( 3 \) (remains the same)

5. **Identify the New Element**:
The element with atomic number 2 is helium (He). Therefore, the notation for the new element is \( ^3_2He \).

6. **Final Answer**:
The mass number of the element obtained when tritium undergoes beta decay is **3**.