Percent Abundance

Question

Lithium has two isotopes ⁶Li and ⁷Li and we found that the most abundant is 92.5%

Identify the most abundant isotope

Answer 1

Looking in the periodic table, we see that the average atomic mass for Li is listed as 6.941. By inspection, this tells us that the most abundant isotope of the two possibilities would be ⁷Li. But, we can also show this mathematically.

If ⁶Li = 92.5 %, that means ⁷Li = 7.5%. If this is the case, we calculate the average atomic mass as....

(0.925)(6) + (0.075)(7) = 5.55 + 0.525 = 6.075

If ⁶Li = 7.5% and ⁷Li = 92.5%, we calculate the average atomic mass as ...

(6)(0.075) + (7)(0.925) = 0.45 + 6.475 = 6.925 (which is much closer to the 6.941 of the periodic table)