Why are Na and K called Sodium and Potassium? (16:9)

Video by KNOW (KnowNow).

The chemical elements “sodium” and “potassium” have symbols “Na” and “K” respectively. They are so common, they were identified relatively early in the history of chemistry, they are everywhere… so why aren’t the symbols “So” and “Po”, for example? “So” is not taken, and “Po” stands now for “polonium” which Marie Curie extracted from the uranium ore about 90 years after potassium got its name. So, why??