## Chemistry and Chemical Reactivity (9th Edition)

Oxygen: $n=10$ Lead: first isotope $n=124$ second isotope $n=122$
According to the periodic table, the atomic mass of oxygen is 8. From the question, oxygen isotope's mass number is 18. The number of neutrons is given by: $A=Z+n\rightarrow 18=8+n$ $n=10$ For lead (Pb), $Z=82$ in the first isotope $A=206\rightarrow n=124$ in the second isotope $A=204\rightarrow n=122$