You have 2 very closely related compounds: Sodium Carbonate and Sodium Bicarbonate. A modern name for Sodium Bicarbonate is Sodium Hydrogen Carbonate.
So, structurally sodium carbonate looks a bit like this (actually there should be 2 Na+ ions):
Sodium carbonate is a stronger base than sodium hydrogen carbonate, and it's all due to the hydrogen tacked to one of the oxygens. Adding another hydrogen makes it an acid: