Which statement best explains why calcium has a larger atomic radius than magnesium?

• A. Calcium has more electron shells than magnesium, increasing the distance between the nucleus and the outermost electrons.
• B. Calcium has a higher nuclear charge, which pulls the electrons closer to the nucleus and decreases the atomic radius.
• C. Magnesium has more protons than calcium, resulting in a larger atomic radius.
• D. Magnesium has more electron shielding than calcium, causing its atomic radius to be larger.

Answer: (A)

Verified step by step guidance

1. Step 1: Understand that atomic radius is influenced by the number of electron shells (energy levels) and the effective nuclear charge experienced by the outermost electrons.
2. Step 2: Recognize that calcium (Ca) and magnesium (Mg) are both in Group 2 of the periodic table, but calcium is in Period 4 while magnesium is in Period 3.
3. Step 3: Note that calcium has more electron shells than magnesium, meaning its outermost electrons are in a higher principal energy level, which increases the distance from the nucleus.
4. Step 4: Consider that although calcium has more protons (higher nuclear charge), the increased number of inner electron shells causes greater electron shielding, reducing the effective nuclear charge felt by the outermost electrons.
5. Step 5: Conclude that the increased number of electron shells and the resulting greater distance between the nucleus and outermost electrons in calcium leads to a larger atomic radius compared to magnesium.