Correct Answer:
B. Lithium
The correct answer is Lithium because it possesses the lowest density and lowest atomic mass of all solid metallic elements on the periodic table.
- Step 1 (Density Analysis): Lithium (Li) is an alkali metal with an exceptionally low density of approximately 0.534 g/cm³. This is so light that a block of pure lithium metal can easily float on top of water or oil.
- Step 2 (Atomic Mass): Positioned at atomic number 3, it has an atomic weight of roughly 6.94, making it structurally the least dense solid element at standard room temperature and pressure.
- Incorrect Options:
- Lead is incorrect because it is a dense, heavy post-transition metal with a high density of 11.34 g/cm³.
- Mercury is incorrect because it is a dense liquid transition metal with a massive density of 13.53 g/cm³.
- Silver is incorrect because it is a precious heavy transition metal with a density of 10.49 g/cm³.