posted by Donneil .
explain why a graph showing the relationship between the amount of a particular element and the element's mass is a straight line?
Mass is a measure of "amount". They mean the same thing. Therefore the relationship is linear.
If you are referng the mass of one atom, or a fixed number (e.g. Avogadro's numbder) of atoms, then of course the "amount" (mass) is proportional to the mass of each atom
Because the energy in the element is at the lowest.