The isotope lithium-7 has a mass of 7.0160 atomic mass units, and the isotope lithium-6 has a mass of 6.0151 atomic mass units. given the information that 92.58 percent of all lithium atoms found in nature are lithium-7 and 7.42 percent are lithium-6, calculate the atomic mass of lithium, li (atomic number 3).

Atomic mass of Li is the weighted average atomic mass with regard to their percentage abundance in nature

atomic mass of Li - mass of Li-6 x % of Li-6 + mass of Li-7 x % of Li-7

substituting these values in the formula

atomic mass of Li - 6.0151 x 7.42% + 7.0160 x 92.58 %

atomic mass of Li - 0.446 + 6.495 = 6.941

therefore atomic mass of Li is 6.941