What is the average atomic mass of hafnium?

A) 176 amu

B) 177 amu

C) 179 amu

D) 180 amu

C) 179 amu. Hf or hafnium is an element having an atomic number of 72. It is a transition metal with at least 34 isotopes, five of these stable isotopes have an atomic mass ranging from of 176 amu to 180 amu. Getting the average atomic mass requires knowledge on the % abundance of the stable isotopes, and then getting the weighted average.