Period 3 trends in ionization energy and melting points

Steps:

• Period 3: Na, Mg, Al, Si, P, S, Cl, Ar; consecutive elements follow atomic number order.
• First ionization energy (IE) generally increases across period but drops at Al (after Mg) and S (after P) due to electron configuration stability.
• Melting points (MP) high for metals (Na-Mg-Al) and Si (network solid), low for P and S (molecular solids).
• For each option, check if middle element (Y) has highest IE and lowest MP among the trio.

Why D is correct:

• In Si-P-S, P (Y) has highest first IE (1012 kJ/mol) from stable half-filled 3p³ configuration and lowest MP (44°C) as white phosphorus forms discrete P₄ molecules.

Why the others are wrong:

• A: Mg has highest IE but MP (650°C) > Na (98°C), so not lowest.
• B: Si has highest IE; Al (Y) has neither (IE 578 kJ/mol, MP 660°C).
• C: P has highest IE and lowest MP, but P is Z, not Y (Si).

Final answer: D