Here's why:

* Elements are the simplest forms of matter and cannot be broken down further by chemical means. Examples include oxygen (O), carbon (C), and sodium (Na).

* Sodium bicarbonate, also known as baking soda, has the chemical formula NaHCO3. This formula tells us that it is made up of three different elements: sodium (Na), hydrogen (H), carbon (C), and oxygen (O).

Therefore, sodium bicarbonate is a compound, formed by the chemical combination of multiple elements.