Which pure substance can be classified as an element

Diamond since its so polished and so strong since the  diamond is simply a special form, or “allotrope”, of carbon. there are several allotropes in the carbon family including carbon nanotubes, amorphous carbon, diamond and graphite. all are made up of carbon atoms, but the types of atomic bonds between them differ which gives rise to different material structures and properties.
so therefore its almost a element 
hope i  

F  you double the  force, you double the acceleration, but  if  you double the  mass, you velocity. all bodies accelerate at the  same  rate regardless of size or  mass.