Q. Which one of the following pairs of metals constitutes the lightest metal and the heaviest metal, respectively? (UPSC Prelims 2008)
Answer: Lithium and osmium
Notes: Lithium (Li) is the lightest metal with a density of 0.534 g/cm³, much lower than aluminium (2.70 g/cm³) or others listed. Osmium (Os) is the densest naturally occurring element and heaviest metal at 22.59 g/cm³, surpassing mercury (13.534 g/cm³). Therefore, lithium and osmium correctly pair the lightest and heaviest metals. The other options are incorrect as they either use heavier lithium alternatives or lighter osmium substitutes.