## Chemistry and Chemical Reactivity (9th Edition)

The atomic weight is the weighted average of the isotope's weights, weighted by their abundances. So for the unknown atom X: The sum of their abundances must be normalized: $14\%+24.1\%+22.1\%+52.4\%=112.6\%$ $w=(203.94\times0.14+205.97\times0.241+206.98\times0.221+207.98\times0.524)\times\frac1{1.126}$ $w=206.84u$ Which is closer to Pb (207.2u).