Among which of these element pairs is the increase in radius minimum?

Options

A. Li to Na
B. Na to K
C. K to Rb
D. Rb to Cs

Answer

To determine among the given pairs of elements (Lithium, Sodium, Potassium, Rubidium, and Cesium) where the increase in atomic radius is minimum, we will analyze the trends in atomic radii within Group 1 (alkali metals) of the periodic table.

### Step-by-Step Solution:

1. **Identify the Elements**: The elements given are Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), and Cesium (Cs). These elements belong to Group 1 of the periodic table.

2. **Understand the Trend**: As we move down a group in the periodic table, the atomic radius generally increases. This is due to the addition of electron shells, which outweighs the effect of increased nuclear charge.

3. **List the Atomic Radii**:
- Lithium (Li): ~152 pm
- Sodium (Na): ~186 pm
- Potassium (K): ~227 pm
- Rubidium (Rb): ~248 pm
- Cesium (Cs): ~262 pm

4. **Calculate the Increase in Radius**:
- From Li to Na: 186 pm - 152 pm = 34 pm
- From Na to K: 227 pm - 186 pm = 41 pm
- From K to Rb: 248 pm - 227 pm = 21 pm
- From Rb to Cs: 262 pm - 248 pm = 14 pm

5. **Identify the Minimum Increase**:
- The increases in atomic radius are:
- Li to Na: 34 pm
- Na to K: 41 pm
- K to Rb: 21 pm
- Rb to Cs: 14 pm
- The smallest increase in atomic radius occurs between Rb and Cs (14 pm).

6. **Conclusion**: The pair with the minimum increase in atomic radius is **Rubidium (Rb) to Cesium (Cs)**.

### Final Answer:
The increase in radius is minimum between Rubidium (Rb) and Cesium (Cs).

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