Not sure on allotropes, don't want to give you the wrong answer so hopefully someone else can help you there.
Isotopes are atoms of the element with the same number of protons, but a different number of neutrons. Say for example, you look at Carbon on a periodic table. Its relative atomic mass is 12.0, and its proton number is 6. For an isotope of carbon, you would see the relative atomic mass change - not the proton number. (as relative atomic mass is just proton and neutrons.)
To sum up isotopes
Relative atomic mass changes.
Proton number does not change.
Answered Sat 5th May, 2012 @ 17:23 by Becky Edited by Becky on Sat 5th May, 2012 @ 17:23
Hiya thank you so much becky,**=D it really was helpful now i juist hope i get an a* on my physics and chemistry exams in the next 3 weeks! =D