Common Electron Count in Stable Ions

Steps:

• Phosphorus (atomic number 15) forms P³⁻ ion by gaining 3 electrons, resulting in 18 electrons total.
• Potassium (atomic number 19) forms K⁺ ion by losing 1 electron, resulting in 18 electrons total.
• Both ions achieve the stable octet configuration of argon (18 electrons).
• Subatomic particles like protons and neutrons remain unchanged by ionization and differ between elements.

Why A is correct:

• Common ions P³⁻ and K⁺ both have 18 electrons to match noble gas stability.

Why the others are wrong:

• B: Neutrons vary by isotope; typical ³¹P has 16 neutrons, ³⁹K has 20.
• C: Protons define atomic number (15 for P, 19 for K).
• D: Not enough information.

Final answer: A