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!

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.