Neutron counts in common isotopes of K and Ar

Steps:

• Atomic number of K is 19 (19 protons); common isotope K-39 has 20 neutrons.
• K⁺ ion retains the K-39 nucleus, so 20 neutrons and 18 electrons.
• Atomic number of Ar is 18 (18 protons); common isotope Ar-40 has 22 neutrons and 18 electrons.
• Compare: Ar has 22 neutrons vs. K⁺'s 20, confirming A.

Why A is correct:

• Ar-40 nucleus contains 22 neutrons, exceeding the 20 neutrons in the K-39 nucleus of K⁺, per standard isotopic data.

Why the others are wrong:

• B: Both K⁺ and neutral Ar have 18 electrons.
• C: K⁺ has 39 nucleons (19p + 20n); Ar has 40 (18p + 22n).
• D: Ar-40 atomic mass (≈40 u) exceeds K-39 (≈39 u); ion mass difference is negligible.

Final answer: A