Identifying Group 1 element using density and reactivity trends

Steps:

• Group 1 elements: Li, Na, K, Rb, Cs, with density generally increasing down the group (Li least dense at 0.53 g/cm³).
• Least dense element is Li, so "not the least dense" excludes Li.
• Reactivity increases down the group due to decreasing ionization energy.
• More reactive than K (third in reactivity order) means X is Rb or Cs.

Why C is correct:

• Not enough information to distinguish Rb from Cs; both are denser than Li and more reactive than K per periodic trends.

Why the others are wrong:

• A. lithium: it is the least dense element in Group 1.
• B. sodium: it is less reactive than potassium.
• D. caesium: it fits both statements, creating ambiguity with Rb.

Final answer: Not enough information.