Identifying L as Na and M as Cl from Period 3 properties

Steps:

• Oxide of L solid with giant structure and chloride unreactive with water identifies L as Na: Na₂O is giant ionic lattice, NaCl dissolves without hydrolysis.
• Only Ar has lower melting point than M identifies M as Cl: Cl₂ has second-lowest mp (-101°C) in Period 3.
• Compound of L (Na) and M (Cl) is NaCl, an ionic solid.
• NaCl mp (801°C) exceeds Cl₂ mp, but fits as the specified compound of L and M.

Why C is correct:

• NaCl is the formula for the ionic compound of Na (L) and Cl (M), per ionic bonding rules for metal-nonmetal.

Why the others are wrong:

• A. Al₂S3 uses Al (whose Cl reacts with water) and S, not L/M.
• B. MgS uses Mg (possible L but M=S has multiple lower mps) and S.
• D. PCl₃ uses P (oxide molecular, not giant; Cl reacts with water) and Cl.

Final answer: C