Le$-19$:

$7$:

Le$-17$:

$2$

Le$-15$:

Le$-19$ has an atomic mass of $19\mathrm{.}0134u,Le-17$ has an atomic mass of $17\mathrm{.}4629u,$ and Le$-15$ has an atomic mass of $15\mathrm{.}7384u.$

Use the natural abundance and atomic mass of each isotope to determine the average atomic mass of Le$.$

average atomic mass