 NEWTON, Ask A Scientist! Name: Hope Status: educator Age: 9-12
Location: CA Country: N/A Date: 12/29/2004  I teach high school
chemistry. I am having problems explaining to a student (and
myself!) why the mass number for a particular isotope is not a
whole number. I understand that the atomic mass is a weighted
average of isotopes and therefore should be a decimal but why
aren't isotopes whole number values for mass if we are using
atomic mass units with protons-1 and neutrons-1? Does it have to
do with nuclear binding forces? I am pretty darn confused about
this. I have another question but I will put it in another box.
Thanks for any/all your help and happy holidays!  The "mass
defect" -- the fact that the mass of an isotope is less that the
sum of its nucleons is due to the fact that when nucleons
combine (either by fusion, for elements with atomic number less
than iron[A=26]; or fission for elements with atomic number
greater than iron) there is a release of energy due to the
nuclear binding force, which