Which of the following species has the largest atomic radius?

• A. Na^+
• B. Na
• C. Mg
• D. Mg^{2+}

Answer: (B)

Verified step by step guidance

1. Recall that atomic radius generally decreases across a period from left to right due to increasing nuclear charge pulling electrons closer, and increases down a group due to the addition of electron shells.
2. Identify the elements and ions given: Na (neutral sodium atom), Na^+ (sodium ion), Mg (neutral magnesium atom), and Mg^{2+} (magnesium ion). All are in the same period (period 3).
3. Understand that cations (positively charged ions) have fewer electrons than their neutral atoms, resulting in a smaller radius because the same nuclear charge is attracting fewer electrons, pulling them closer.
4. Compare the species: Na^+ has lost one electron compared to Na, and Mg^{2+} has lost two electrons compared to Mg, so both ions will be smaller than their neutral atoms. Between Na and Mg, Mg has a higher nuclear charge, so Mg will have a smaller radius than Na.
5. Conclude that among the given species, the neutral sodium atom (Na) has the largest atomic radius because it has more electrons than Na^+ and Mg^{2+}, and a lower nuclear charge than Mg, resulting in a larger size.