Isoelectronic particles share the same electron count

Steps:

• Identify atomic numbers: Ar=18, Ca=20, Na=11, K=19, Fe=26, Ca=20, S=16.
• Calculate electrons: Neutral atoms have electrons equal to atomic number; cations lose electrons (Na+ =11-1=10, K+=19-1=18, Fe2+=26-2=24, Ca2+=20-2=18); anions gain electrons (S2-=16+2=18).
• Compare pairs: Ar (18) vs Ca (20); Na+ (10) vs K+ (18); Fe (26) vs Fe2+ (24); Ca2+ (18) vs S2- (18).
• Select pair with identical electron numbers.

Why D is correct:

• Ca2+ and S2- are isoelectronic, both with 18 electrons (Ca atomic number 20 minus 2 electrons; S atomic number 16 plus 2 electrons).

Why the others are wrong:

• A: Ar has 18 electrons; Ca has 20.
• B: Na+ has 10 electrons; K+ has 18.
• C: Fe has 26 electrons; Fe2+ has 24.

Final answer: D