Period 3 trends in ionization energy and melting points identify Y as phosphorus

Steps:

• Period 3 elements are Na to Ar; select consecutive trios from options.
• First ionization energy (IE) rises across period with exceptions; melting point (MP) peaks for metals, drops for non-metals.
• For Si-P-S, IE values: Si (786 kJ/mol), P (1012 kJ/mol), S (1000 kJ/mol); P has highest IE.
• MP values: Si (1414°C), P (44°C), S (113°C); P has lowest MP, matching Y.

Why D is correct:

• Phosphorus (Y) exhibits the highest first IE due to stable half-filled p subshell and lowest MP as molecular P4.

Why the others are wrong:

• A: Mg has highest IE but MP (650°C) exceeds Na's (98°C).
• B: Si has highest IE but highest MP (1414°C).
• C: Identical to D, but assuming variant like P-S-Cl, S lacks highest IE (Cl=1251 kJ/mol).

Final answer: D