Question

select the correct answer.
which atom or ion is the largest?
a. k
b. k⁺
c. ca
d. ca²⁺
e. li

Explanation:

To determine the largest atom/ion, we analyze atomic/ionic radii trends:

1. Ion vs. Atom (Same Element): For a given element, cations are smaller than the neutral atom (e.g., \( \text{K}^+ < \text{K} \), \( \text{Ca}^{2+} < \text{Ca} \)) because losing electrons reduces electron - electron repulsion, and the nucleus pulls remaining electrons closer.
2. Atoms (Different Elements): Atomic radius generally increases down a group (due to adding electron shells) and decreases across a period (due to increasing nuclear charge, pulling electrons closer).

• \( \text{Li} \) is in period 2, \( \text{K} \) and \( \text{Ca} \) are in period 4. So \( \text{Li} \) is smaller than \( \text{K} \) and \( \text{Ca} \).
• Between \( \text{K} \) (atomic number 19, electron configuration \( [\text{Ar}]4s^1 \)) and \( \text{Ca} \) (atomic number 20, electron configuration \( [\text{Ar}]4s^2 \)): Across a period, as nuclear charge increases, atomic radius decreases. So \( \text{K} \) has a larger atomic radius than \( \text{Ca} \) because \( \text{K} \) has a smaller nuclear charge (19 protons vs. 20 protons in \( \text{Ca} \)) and the same valence shell (4s), so the electrons in \( \text{K} \) are less strongly pulled by the nucleus.

Combining these: \( \text{K} > \text{K}^+ \), \( \text{Ca} > \text{Ca}^{2+} \), \( \text{K} > \text{Ca} \), and \( \text{K} > \text{Li} \). So \( \text{K} \) is the largest.

Answer:

A. K