What differentiates atoms from elements?

Oxygen (for example) is an element (pure atom) while an atom can be more than one element (an atom of H2O for example is more than one element), and there can be atoms of an element (like an atom of Oxygen).

A particular atom will have the same number of protons and electrons and most atoms have at least as many neutrons as protons. An element is a substance that is made entirely from one type of atom. For example, the element hydrogen is made from atoms containing just one proton and one electron.

My only guess is obtain a metal and heat it in a boiling water bath (of known temperature) this will be your initial temperature. Now obtain a calorimeter cup with water of known temperature as well. Place the metal into the calorimeter cup and record the temperature after 5 minutes. You now have delta T, mass of the metal, and Q. Solve for C.

Hope this helps xox :)