Review the periodic table entry for lithium (Li). Lithium has two major isotopes. Which of the following statements best explains the reported average atomic mass of lithium? The isotope lithium- 7 is more common than the isotope lithium-6. Lithium-9 is about twice as abundant as lithium- 7. Lithium-7 and lithium-6 exist in approximately equal proportions. The two major isotopes of lithium have 6 and 8 neutrons