Why is baking soda a ionic compound?

The formula for baking soda is NaHCO3, or sodium bicarbonate. Bicarbonate is a polyatomic ion with a charge of - 1 just because of the way it's structured (we haven't learned exactly how it bonds because the goal of ionic compounds is usually to get a neutral charge; polyatomic ions are just ones you have to memorize). All polyatomic ions bond ionically (aka they give/take electrons to get to a full outer shell of electrons, aka have 0 valence electrons). Since sodium (Na) has an atomic charge of + 1, it can "give" an electron to sodium bicarbonate and both are now neutrally charged and bonded ionically.

Just a side note for once you understand the fundamentals of ionic compounds: all polyatomic ions bond ionically, and all compounds involving a metal and a nonmetal are ionic (anything to the left of the "staircase" and anything to the right of the "staircase"). Hydrogen is an exception and can act as a metal or a nonmetal. Any two nonmetals form a molecular compound (they're bonded covalently), and that isn't based on charge. Best of luck!