What is the average atomic mass of the element? an element, x, has two isotopes, x-14 and x-15. use the data in the table to find the average atomic mass of element x. isotope atomic mass abundance (%) x-14 14.003 99.636 x-15 15.000 0.364 the average atomic mass of element x is blank amu.

The average atomic mass of element X is 14.007 u.

The average atomic mass of X is the weighted average of the atomic masses of its isotopes.

We multiply the atomic mass of each isotope by a number representing its relative importance (i.e., its % abundance).

Thus,  

0.996 36 × 14.003 u =13.952 03 u

0.003 64 × 15.000 u = 0.054 60 u

TOTAL = 14.007     u