Considering the definition of atomic mass, isotopes and atomic mass of an element, the average mass of lithium is 6.96.
First of all, the atomic mass (A) is obtained by adding the number of protons and neutrons in a given nucleus of a chemical element.
The same chemical element can be made up of different atoms, that is, their atomic numbers are the same, but the number of neutrons is different. These atoms are called isotopes of the element.
On the other hand, the atomic mass of an element is the weighted average mass of its natural isotopes. In other words, the atomic masses of chemical elements are usually calculated as the weighted average of the masses of the different isotopes of each element, taking into account the relative abundance of each of them.
In this case, Lithium-6 is 4% abundant (the atomic mass is 6) and lithium-7 (the atomic mass is 7) is 96% abundant. Then, the average mass of lithium can be calculated as:
6×0.04 + 7×0.96= 6.96
Finally, the average mass of lithium is 6.96.
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