Trends in Group I, VII, and noble gas properties

Steps:

• Evaluate A: Noble gases unreactive due to full valence shells, but helium has 2 electrons, not 8.
• Evaluate B: Astatine as heaviest halogen follows trend of increasing boiling points down Group VII, predicting solid state.
• Evaluate C: Group I reactivity increases down group due to easier electron loss; Group VII reactivity decreases due to weaker oxidizing power.
• Evaluate D: Chlorine, stronger oxidant than bromine, displaces bromide ions, producing colored Br2.

Why B is correct:

• Halogens' physical states trend from gas to solid down the group due to increasing atomic mass and van der Waals forces; astatine (At, atomic number 85) extrapolates as a black solid like iodine.

Why the others are wrong:

• A: Helium has 2 valence electrons, violating the octet rule claim.
• C: Group VII reactivity decreases down the group as electron affinity weakens.
• D: Displacement reaction Cl2 + 2Br- → Br2 + 2Cl- produces brown bromine color.

Final answer: B