Periodic trends in metallic character across groups

Steps:

• Groups indicate valence electrons: gallium (Group III) has 3, selenium (Group VI) has 6.
• Both elements are in period 4, so compare horizontal position in the periodic table.
• Metallic character decreases from left to right across a period due to increasing electronegativity.
• Thus, gallium (left, Group III) is metallic, while selenium (right, Group VI) is non-metallic.

Why D is correct:

• Non-metallic character increases across a period (from group 13 to 16), as defined by the periodic trend where elements gain more valence electrons and higher electronegativity, making selenium more non-metallic than gallium.

Why the others are wrong:

• A: Atomic numbers show selenium (34 protons) has more than gallium (31); group info alone doesn't predict protons.
• B: Both form ions, but gallium (metal) loses electrons while selenium (non-metal) gains; group info doesn't clearly favor one over the other in likelihood.
• C: Group numbers directly show selenium has more valence electrons (6 vs. 3).

Final answer: D