There isn't a single, common molecule that specifically consists of 21 carbon atoms. Molecules with 21 carbon atoms can be incredibly diverse, ranging from simple hydrocarbons to complex biomolecules.

To help you narrow down what you're looking for, I need some more information.

Could you tell me more about what you're interested in? For example:

* What kind of molecule are you looking for? (e.g., a hydrocarbon, a protein, a carbohydrate, etc.)

* What is the context of your question? (e.g., are you studying a specific chemical reaction, or are you just curious about molecules with this number of carbon atoms?)

Once I have a better understanding of what you're looking for, I can give you a more helpful answer.