There are four radioactive decay series called thorium `(4n)`, uranium `(4n + 2)` actinium `(4n + 3)` and neptunium `(4n + 1)` series. Neptunium series is artificial while other three series are natural. The end productsd of each radioacitve decay series have stable nuclei. All natural decay series terminate at lead but neptunium or artificial series terminate at bismuth.
Actinium series begins with an isotope of

Options

A. Actinium
B. Radium
C. Uranium
D. Polonium

Answer

To solve the question regarding the starting isotope of the actinium series, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Series**:
The actinium series is classified as a decay series represented by the formula \(4n + 3\). This means that any isotope in this series, when its mass number is divided by 4, will leave a remainder of 3.

2. **Determine the End Product**:
The actinium series is a natural decay series that ultimately terminates at a stable isotope of lead. The stable isotope of lead that it ends with is lead-207, which has an atomic number of 82.

3. **Find the Starting Isotope**:
To identify the starting isotope of the actinium series, we need to find an isotope whose mass number, when divided by 4, gives a remainder of 3.

4. **Check Common Isotopes**:
The most commonly known isotopes that are associated with the actinium series include:
- Uranium-235 (U-235)
- Thorium-232 (not part of the actinium series)
- Actinium-227 (the element itself)

5. **Calculate the Remainder**:
- For Uranium-235:
\[ 235 \div 4 = 58 \quad \text{remainder} \quad 3 \]
- For Actinium-227:
\[ 227 \div 4 = 56 \quad \text{remainder} \quad 3 \]
Both isotopes yield a remainder of 3 when divided by 4, indicating they belong to the \(4n + 3\) series.

6. **Conclusion**:
The actinium series begins with Uranium-235, which is the most commonly recognized starting point for this series.

### Final Answer:
The actinium series begins with an isotope of **Uranium** (specifically Uranium-235).

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