Which one among the following metals is not stored under oil?

The correct answer is Option 4 (Lithium). While all the listed elements are highly reactive alkali metals that belong to Group 1 of the periodic table, their physical properties dictate different storage methods.

Sodium (1), Rubidium (2), and Potassium (3) are extremely reactive and catch fire when exposed to air or moisture. They have a density higher than kerosene or paraffin oil, allowing them to sink and remain fully submerged, protected from oxidation. Therefore, they are traditionally stored under oil.

Lithium (4) is the exception because it is the lightest known metal. Its density is approximately 0.534 g/cm³, which is significantly lower than that of standard paraffin oil or kerosene (approx. 0.8 g/cm³). Consequently, Lithium floats on the surface of the oil, bringing it into contact with atmospheric oxygen and moisture. To ensure safety, Lithium is instead stored by wrapping it in paraffin wax or keeping it in an airtight container under an inert atmosphere.