Periodic Table Group Properties

Steps:

• Recall trends in Group VII (halogens): physical state changes from gas to liquid to solid down the group due to increasing atomic size and van der Waals forces.
• Evaluate option A: noble gases are unreactive with full outer shells, but helium has 2 electrons, not 8.
• Evaluate option C: Group I reactivity increases down the group (larger atoms lose electrons easier), but Group VII reactivity decreases (weaker oxidizing power).
• Evaluate option D: chlorine displaces bromide from solution, producing brown Br2 color.

Why B is correct:

• Halogens transition to solids down Group VII; iodine is gray solid, so astatine, below iodine, is a solid, predicted black from its radioactive, heavy nature per periodic trends.

Why the others are wrong:

• A: Helium has 2 valence electrons, not 8, though unreactive.
• C: Group VII reactivity decreases down the group as oxidizing strength weakens.
• D: Chlorine oxidizes bromide to brown bromine, causing color change (Cl2 + 2Br- → 2Cl- + Br2).

Final answer: B