Here's why:

* Concrete nouns refer to things that can be perceived by the senses (sight, touch, smell, taste, hearing).

* Abstract nouns refer to ideas, concepts, qualities, and states of being.

Molecules, even though very small, are physical entities that exist in the real world. You can't see them with the naked eye, but they are made up of atoms and have a definite structure.

Let me know if you have any other noun questions!