X is sodium, forming NaCl with chlorine (Y) Steps: - Period 3 elements include Na, Mg, Al, Si, P, S, Cl; focus on those with solid giant oxide structures (ionic or covalent lattices): Na₂O, MgO, Al₂O₃, SiO₂. - Eliminate Al and Si: their chlorides (AlCl₃, SiCl₄) hydrolyze violently with water, producing HCl. - Remaining: Na and Mg; both have stable chlorides (NaCl, MgCl₂) that dissolve without reaction, but X is Na as the group 1 metal with purely ionic giant Na₂O and non-hydrolyzing NaCl. - Y is Cl (period 3 halogen); compound is 1:1 ionic NaCl. Why C is correct: - NaCl follows the ionic bonding rule for group 1 metal and group 17 non-metal, matching X=Na properties. Why the others are wrong: - A: Al₂S₃ requires X=Al, but AlCl₃ reacts with water via hydrolysis (AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl). - B: MgS requires X=Mg, but Mg fits less precisely as group 2; problem specifies properties aligning with group 1 Na. - D: PCl₃ requires X=P, but P₄O₁₀ has molecular (not giant) structure and PCl₃ hydrolyzes (PCl₃ + …