The atomic mass of an element, such as lithium, refers to the weighted average of the masses of its naturally occurring isotopes. In the case of lithium, it has two stable isotopes: lithium-6 (^6Li) and lithium-7 (^7Li).
The atomic mass of lithium is commonly listed as 6.941 atomic mass units (amu) on the periodic table. This value is an average that takes into account the relative abundances of lithium-6 and lithium-7 in nature.
Lithium-6 has a mass of approximately 6.0151 amu, and lithium-7 has a mass of approximately 7.0160 amu. The higher abundance of lithium-7 in nature contributes more to the average atomic mass of lithium, resulting in a value closer to 7 than 6.
So, while lithium-6 and lithium-7 are the two isotopes of lithium, the atomic mass of lithium is approximately 6.941 amu.