Sodium is a chemical element, number 11 on the periodic table. It had the atomic weights of 11 and 19.
In 2369, sodium was on the Table of Elements twice, once under the symbol "Na", and again under the symbol "K". (TNG: "Rascals")
In reality, sodium appears only once on the periodic table, under the symbol "Na", whereas "K" is actually the atomic symbol for potassium.