Not only are lead nuclei heavier than gold ones (207 vs 200), but the atoms are smaller (ionic radius 119pm vs 137pm). There is every reason for Pb to be heavier. Why is it lighter? (11.34 vs 19.31 g/cm^3) They both have a FCC crystal structure, so the packing should be equally efficient. Source: https://courses.edx.org/courses/course-v1:MITx+3.091x+3T2016/periodic_table/ (unfortunately, you probably need an edx account to visit) Ignorance is lead, your answer will be gold to me. Thanks!