Chemistry

Question Analysis
This question involves a direct comparison of two adjacent elements in the same period and requires justifying the radius difference.
The main focus is on linking atomic structure (protons, electrons) to periodic trends.

Key Concept Explanation
Potassium (Group 1) and calcium (Group 2) are in Period 4. Ca has one more proton than K, creating a higher nuclear charge. This stronger charge pulls the valence electrons (in the same shell) closer, resulting in a smaller atomic radius for Ca.

Step-by-Step Solution
1. Confirm the period: K and Ca are in Period 4 (same valence shell, n=4).
2. Compare nuclear charge: Ca (atomic number 20) has more protons than K (atomic number 19).
3. Apply the period trend: Higher nuclear charge → smaller atomic radi...