- isotopes are variations of the same [[Element]] but with different # of [[Neutrons]] - each isotope variation is based on atomic weight - atomic weight = mass of atom / mass of main [[Isotope]] of [[Hydrogen]] - most abundant isotope of [[Hydrogen]] has 0 [[Neutrons]] http://www.infocobuild.com/education/audio-video-courses/physics/Physics10-Spring2015-Berkeley/lecture-03.html