To determine the half-life of the longest-lived isotope of Francium, we can follow these steps:
### Step 1: Identify the Element
- Francium is an alkali metal with the atomic number 87.
**Hint:** Remember that the atomic number helps identify the element in the periodic table.
### Step 2: Determine the Longest-Lived Isotope
- The longest-lived isotope of Francium is Francium-223 (₈₇Fr²²³).
**Hint:** Isotopes of an element have the same number of protons but different numbers of neutrons, which affects their stability.
### Step 3: Research the Half-Life
- The half-life of Francium-223 is known to be approximately 21 minutes.
**Hint:** The half-life is the time required for half of the radioactive nuclei in a sample to decay.
### Step 4: Conclusion
- Therefore, the half-life of the longest-lived isotope of Francium (Francium-223) is 21 minutes.
**Final Answer:** The correct option is A: 21 minutes.
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