Question

in which pair does the first atom or ion have a larger radius than the second atom or ion?
na, rb
br, br-
k, k+
s, mg

Explanation:

• For \(Na\) and \(Rb\): As we move down a group (both are in Group 1), atomic radius increases. So \(Rb\) has a larger radius than \(Na\).
• For \(Br\) and \(Br^{-}\): When an atom gains an electron to form an anion (\(Br^{-}\)), electron - electron repulsion increases. But the first atom (\(Br\)) does not have a larger radius than the second (\(Br^{-}\)).
• For \(K\) and \(K^{+}\): When an atom loses an electron to form a cation (\(K^{+}\)), the effective nuclear charge per electron increases (since there are fewer electrons). So the radius of \(K\) is larger than that of \(K^{+}\).
• For \(S\) and \(Mg\): As we move across a period (from left to right), atomic radius decreases. \(Mg\) is to the left of \(S\) in Period 3. So \(Mg\) has a larger radius than \(S\).

Answer:

C. \(K, K^{+}\)