Which of the following has the largest atomic or ionic radius?

• A. K
• B. K^+
• C. Ca^{2+}
• D. Ca

Answer: (A)

Verified step by step guidance

1. Step 1: Understand that atomic and ionic radius generally depends on the number of protons (nuclear charge) and the number of electrons (electron cloud size). More protons pull electrons closer, reducing radius; more electrons or electron shells increase radius.
2. Step 2: Identify the species given: neutral potassium (K), potassium ion (K^+), neutral calcium (Ca), and calcium ion (Ca^{2+}). Note their electron configurations and number of electrons.
3. Step 3: Recognize that K and Ca are in the same period (period 4), but Ca has more protons (20) than K (19), so Ca's electrons are pulled in more tightly, making Ca smaller than K.
4. Step 4: For ions, K^+ has lost one electron compared to neutral K, reducing electron-electron repulsion and allowing the remaining electrons to be pulled closer, decreasing radius compared to neutral K. Similarly, Ca^{2+} has lost two electrons, making it even smaller than neutral Ca.
5. Step 5: Conclude that among K, K^+, Ca, and Ca^{2+}, the neutral potassium atom (K) has the largest radius because it has the least effective nuclear charge per electron and the same number of electron shells as the others.