Why is baking soda a compound?
Baking soda is a compound because it's composed of molecues that are all identical. It has the chemical formula `NaHCO_3` and the chemical name sodium hydrogen carbonate. (It was called sodium bicarbonate under an older naming system.)
Elements and compounds are both pure substances, having all identical particles. Elements are composed of atoms, or in a few cases molecules made of only that element (Like `O_2` ). Compounds have definite proportions - there's always one Na, one H, one C and three O's per unit of sodium hydrogen carbonate.
The other classification of matter is mixtures. A mixture contains different types of particles so they can be combined in various proportions. Baking powder is a mixture because it contains both sodium hydrogen phosphate and potassium hydrogen tartrate - two different compounds.