First, find the relative atomic mass of Lead. Since protons and neutrons weigh almost the same, you can consider them each one to have 1 unit (u) of mass. Now, sum the protons and neutrons and let them be units of mass. Do that for the other isotopes too. read more
Cobalt (Co) has a smaller atomic mass than Lead (Pb). Cobalt's atomic mass is only 58.9332 while lead's is 207.2. read more