Subatomic particles in Ce-140 ion

Steps:

• Atomic number 58 means 58 protons in the nucleus.
• Total nucleons 140 = protons + neutrons, so neutrons = 140 - 58 = 82.
• Ce ion is Ce^{4+}, a common oxidation state, so electrons = 58 - 4 = 54.
• Thus, one Ce ion has 58 protons, 82 neutrons, 54 electrons.

Why A is correct:

• Matches definitions: protons = atomic number (58); neutrons = mass number - atomic number (140 - 58 = 82); electrons = protons - charge (58 - 4 = 54 for Ce^{4+}).

Why the others are wrong:

• B: Incomplete or missing choice, provides no valid numbers.
• C: Swaps protons and neutrons (82 protons incorrect; atomic number is 58).
• D: Lists neutrons first, then protons, with 62 electrons implying Ce^{4-} (uncommon for Ce).

Final answer: A