Good question! I would say that the strongest element is carbon when it is in the form of diamond. Carbon atoms can bond to each other in different ways to form different structures – we call these allotropes. One of these allotopes is diamond. Diamond is the hardest known natural substance and for that reason it is often used in drill bits.
For the weakest element, I would probably go for helium – one of the noble gases. It is very light and unreactive.
Scientists have fairly recently discovered a material called ‘graphene’ that now holds the record for strongest elemental material (a different form of carbon to diamond – it exists in sheets made up of hexagons of carbon atoms). To give you an idea, a sheet of graphene the same thickness as clingfilm would take the weight of a medium sized car to break it!