Lead has four stable isotopes (listed with their atomic mass and abundance, respectively): ²⁰⁴Pb (203.9730 u, 1.4%), ²⁰⁶Pb (205.9745 u, 24.1%), ²⁰⁷Pb (206.9759 u, 22.1%), and ²⁰⁸Pb (207.9767 u, 52.4%). Calculate the atomic mass of lead.

Copper has two naturally occurring isotopes: Cu-63 and Cu-65. Cu-63 has a natural abundance of 69.15%. Calculate the mass of Cu-63 if the mass ratio of the two isotopes is 1.0318.

Calculate the relative abundances of the isotopes of boron if Boron-10 has a mass of 10.0129 amu and Boron-11 has a mass of 11.0093 amu.

In the periodic table, Silicon has an atomic mass listed as 28.08550 and not 28.00000. Choose the statement that explains why atomic masses in the periodic table are not integral numbers. 

Lithium has only two naturally occurring isotopes. The mass of lithium-6 is 6.01512 amu and the mass of lithium-7 is 7.01601 amu. Calculate the relative abundances of the two isotopes.