2. Which of the following correctly lists the five atoms in order of increasing size (smallest to largest)? *
OA) Br< Pb < At < Cs < Fr
OB) Br < Cs < At < Pb < Fr
OC) Br < Cs < Pb < At < Fr
OD) Br < At < Pb < Cs E) Br < Pb < Cs < At < Fr

To calculate the average atomic weight, each exact atomic weight is multiplied by its percent abundance, then add the results together and round off to an appropriate number of significant figures.  So according to what's given, the average atomic mass of element M should be (23.9850*0.7899)+(24.9858*0.1000)+(25.9826*0.1101)=24.30