The number of isotopes of Cerium is 8 but the isotopes `Ce^140` and `Ce^142` are present in considerable quantities, hence, you need to evaluate what is the most abundant isotope between `Ce^140` and `Ce^142` .
You need to consider the atomic mass of Ce from periodic table, such that:
Atomic mass `= 140.11`
You need to use the following relation to evaluate te most abundant isotope, such that:
`(1-x)*mass Ce^142 + x*mass Ce^140 = 140.11`
`(1-x)*142 + x*140 = 140.11`
`142 - 142x + 140x = 140.11`
`-2x = -1.89 => x = 1.89/2 = 0.945`
Hence, evaluating the most abundant isotope of Cerium, considering only the `Ce^140` and `Ce^142` isotopes, yields `Ce^140` is the most abundant isotope.