Answer
The difference between 210-Bi and 210-Pb is the number of protons each element contains within it's nucleus.
Work Step by Step
Bismuth has 83 protons while Lead only contains 82 protons. The number of protons determines an element's identity. What gives these two elements the same molecular mass of 210 is their number of neutrons. There are 127 neutrons in Bismuth and 128 neutrons in Lead.
