The percent compositions for the naturally occurring isotopes of lithium are as follows: lithium-6 (7.5%), lithium-7 (92.5%), and lithium-8 (0.01%). The average atomic mass of lithium is calculated by taking into account the masses of all of its naturally occurring isotopes and the percent abundance of each isotope. When we look at the average atomic mass of lithium and compare it to the mass numbers of its isotopes, we can see that the average atomic mass is not equal to any of the individual isotope mass numbers. This is because the average atomic mass is a weighted average that takes into account the percent abundance of each isotope. Therefore, the average atomic mass of lithium is not the same as the mass number of any of its individual isotopes.