Group 1 trends: density increases and reactivity increases down the group

Steps:

• In Group 1, reactivity increases down the group due to decreasing ionization energy from larger atomic size.
• Density increases down the group as atomic mass rises faster than atomic volume.
• Caesium (Cs, atomic number 55) is below rubidium (Rb, atomic number 37) in the group.
• Thus, Cs has higher density (1.93 g/cm³ vs. 1.53 g/cm³) and greater reactivity than Rb.

Why A is correct:

• Periodic trends dictate that descending Group 1 lowers first ionization energy (Cs: 376 kJ/mol vs. Rb: 403 kJ/mol), boosting reactivity, while mass increase elevates density.

Why the others are wrong:

• B: Incorrect, as Cs exceeds Rb in reactivity due to easier electron loss.
• C: Incorrect, as Cs has greater density from higher atomic mass.
• D: Incorrect, as Cs surpasses Rb in both properties.

Final answer: A