The element lanthanum has an atomic weight of 139 and consists of two stable isotopes lanthanum-138 and lanthanum-139.
The isotope lanthanum-138 has an atomic mass of 138 amu and a percent natural abundance of 8.90E-2 %.
The isotope lanthanum-139 has a percent natural abundance of 99.9 %.
What is the atomic mass of lanthanum-139?