Question about the definition of an isotopes My answer was as follows : atoms of the same element with differrnt relative atomic masses.

However in the mark scheme the answer was simply : atoms of the same element with different masses.

Was there something wrong with my answer? 

Why does it matter?
Usually the term "relative atomic mass" for an element is a weighted average of the atomic masses of the various isotopes of an element, with the weighting determined by their abundance. So, since the atomic masses of various isotopes enter into this determination, your first answer is not too good.