Atomic Radius Trend in Alkali Metals

The question asks to identify the element with the largest atomic radius among Potassium (K), Rubidium (Rb), Lithium (Li), and Sodium (Na).

Understanding Atomic Radius Trends

Atomic radius refers to the size of an atom, typically measured as the distance from the nucleus to the outermost electron shell. In the periodic table, atomic radius follows predictable trends:

• Across a Period (Left to Right): Atomic radius generally decreases because the number of protons in the nucleus increases, pulling the electrons closer.
• Down a Group (Top to Bottom): Atomic radius generally increases because atoms gain more electron shells as you move down a group, placing the outermost electrons farther from the nucleus.

Analyzing the Elements

The elements provided (Li, Na, K, Rb) are all alkali metals, belonging to Group 1 of the periodic table. Let's look at their positions:

• Lithium (Li) is in Period 2.
• Sodium (Na) is in Period 3.
• Potassium (K) is in Period 4.
• Rubidium (Rb) is in Period 5.

Applying the Trend

Since all these elements are in the same group (Group 1), the trend of increasing atomic radius down the group applies. This means the element located in the lowest period will have the largest atomic radius.

Comparing their positions:

• Li (Period 2)
• Na (Period 3)
• K (Period 4)
• Rb (Period 5)

Following the trend, the atomic radius increases in the order:

Li < Na < K < Rb

Conclusion

Rubidium (Rb), being in Period 5, has the largest number of electron shells among the given options, making it the element with the largest atomic radius.