# What is the average atomic mass of lithium?

The major isotopes of lithium are (i) ""^7Li, and (ii) ""^6Li. The weighted average of these isotopes is $6.941$ $g \cdot m o {l}^{-} 1$.
You will have to look up the isotopic distribution of lithium nuclei. ALL lithium nuclei have 3 positively charged nuclear particles, protons, i.e. $Z$ $=$ $3$. These 3 particles give rise to the element's identity. If you take the weighted average of ALL lithium nuclei, we get an average mass of $6.941$ $g \cdot m o {l}^{-} 1$.