為何鑽石沒有獨立的分子存在?

Diamond is made up of carbon atoms. Each carbon atoms in diamond would form strong covalent bonds with other 4 carbon atoms tetrahedrally. (A carbon atom could form 4 bonds with other atoms at maximum.) This causes giant covalent structure. This structure restricts the motion of carbon atoms in diamond. Thus diamond is hard.

Futhermore, when some energy is applied to a diamond, due to its hardness, the structure of diamond is either (i) remains unchanged (The energy applied is not large enough to break the bonds between each carbon atoms) or (ii) is broken (The energy applied is large enough to break the bonds between carbon atoms). In situation (ii), the carbon atoms are seperated from each other. The diamond structure has been destroyed, and thus the "diamond" is no longer a diamond, but only carbon atoms instead.

As a conclusion, diamond doesn't exist as discrete molecules.

Diamond is made up of C atom. it is a giant covalent structure. Its structure is one Carbon bond to 4 other Carbon atom, linking up to form a network. So there is no discrete molecules of diamond.