A molecule is the smallest piece of a substance that is still that particular substance. For example, you know that salt is a compound made of sodium (Na) and chlorine (Cl). If you took salt apart, you would end up with one atom of Na and one atom of Cl, but it wouldn’t be salt anymore. The smallest particle of salt that is possible is a molecule of salt, made of one atom of Na and one atom of Cl. There are millions of molecules of salt in one grain of salt.

Molecules are made of one or more atoms. Molecules of compounds like sugar, salt, and baking soda are made of two or three different elements. Some of the elements have more than one atom in the molecule. For example, we know there are four elements in one molecule of baking soda (NaHCO₃). To find out how many atoms are in one molecule of baking soda, just count them up. There is one sodium (Na) atom, one hydrogen (H) atom, one carbon (C) atom, and three oxygen (O) atoms. There are a total of six atoms in one molecule of baking soda.