Among the following which element is the rarest of all ?

Options

A. Na
B. Cs
C. Fr
D. K

Answer

To determine which element is the rarest among the given options (sodium, cesium, francium, and potassium), we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Elements**: The given options are:
- Sodium (Na)
- Cesium (Cs)
- Francium (Fr)
- Potassium (K)

2. **Classify the Elements**: All these elements belong to the alkali metals group, which is Group 1 of the periodic table.

3. **Research Rarity**: To find out which of these elements is the rarest, we need to look at their abundance in the Earth's crust:
- Sodium (Na) is quite abundant.
- Potassium (K) is also relatively abundant.
- Cesium (Cs) is less abundant than sodium and potassium but still found in significant quantities.
- Francium (Fr) is extremely rare and is considered the second rarest naturally occurring element.

4. **Consider Radioactivity**: Francium is a radioactive element with a very short half-life, which contributes to its rarity. It is estimated that there are only about 30 grams of francium in the Earth's crust at any given time.

5. **Conclusion**: Based on the information gathered, francium (Fr) is the rarest element among the options provided.

### Final Answer:
**Francium (Fr) is the rarest of all the given elements.**

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