Great question! The answer to this can be confusing. First, let's define what an element, compound, and mixture is.

An element is a type of atom defined by the charge of it's nucleus (the number of protons it has). All the elements can be found on the periodic table of elements ( periodic table ). Examples of elements are hydrogen, oxygen, and carbon. Combinations of atoms bonded together (think of them as joined together) are called molecules.

Molecules that consist of more than one type of element are called compounds. Examples of compounds, and therefore also molecules, are water (H₂O), table salt (NaCl), and carbon dioxide (CO₂). Finally, mixtures are made of two or more types of molecules. Salt water is a common mixture and so is the air that we breathe.

So this brings us back to your question, where does diamond fall into these categories? Diamond is made up entirely of carbon atoms. Since carbon is one element, it is not a compound or a mixture. Still, even though it is made of only one type of element, it wouldn't be fair to call it an element because diamond is really many carbon atoms bonded together in a very particular shape. Carbon can bond together in many different shapes, and when it does it has very different properties. The two most well known would be graphite (the gray, center part of your pencil) which is rather soft allowing you to write with it anddiamond which is very hard. It can also form ball shapes known as "buckminster fullerenes".

Drawings of the different forms of carbon can be found at forms of carbon. So to answer your question, it would be most proper to classify diamond as a molecule, but not a compound as compounds require more than one type of element to bond together.

I hope this helps answer your question!