Why should the stopper of a chemical reagent container be handled carefully?

Since the stopper may have contact with the chemical inside the reagent bottle, we should handle it carefully to prevent contamination by contacting it with other checmicals. In addition, we may not want to get pose our health or body at risk by contacting with hazardous checmicals remainings on the stopper.

All the elements are arranged by melting points on the periodic table. in row they go from metals to gasses. in the row from sodium to argon, sodium is more reactive than the one on its right, magnesium.