720.12 = 207.2012 100 hium has two naturally occurring isotopes. Lithium-6 has an atomic mass of 6.015 amu; lithium-7 ha: omic mass of 7.016 amu. The atomic mass of lithium is 6.941 amu. What is the percentage of natura curring lithium-7? (Make Li-6's percent abundance x and Li-7's percent abundance 1-x)Lithium has two naturally occurring isotopes. Lithium-6 has an atomic mass of 6.015 amu; lithium-7 has an atomic mass of 7.016 amu. The atomic mass of lithium is 6.941 amu. What is the percentage of naturally occurring lithium-7? (Make Li-6's percent abundance x and Li-7's percent abundance 1-x)