# Percent abundance of isotopes

i am trying to figure out the percent abundance of two isotopes. i know how to set up the problem but i dont know how to find the weight of each isotopes. here is the problem:

Cerium (58Ce) has two commonly occuring isotopes, 140Ce and 142Ce. what is the approximate percent abundance of 140Ce?

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Ygggdrasil
Gold Member
2019 Award
140Ce means that the isotope has an atomic mass of (approximately) 140 amu.

Astronuc
Staff Emeritus
i am trying to figure out the percent abundance of two isotopes. i know how to set up the problem but i dont know how to find the weight of each isotopes. here is the problem:

Cerium (58Ce) has two commonly occuring isotopes, 140Ce and 142Ce. what is the approximate percent abundance of 140Ce?
What is the given mass of natural cerium? The mass should be a weighted average based on the isotopic abundances and the mass of each isotope. For two isotopes, it's pretty simple.

Hi kceddis-
There are an abundance of tables on isotopes and nuclear data at LBL (Lawrence Berkeley Laboratory) follow your google search by site:lbl.gov

Here is a table for finding atomic masses of all the isotopes.
http://ie.lbl.gov/toi2003/MassSearch.asp
In this case, type in the elemend Z# or symbol, and it gives the atomic masses of all the isotopes of that element. The masses are given in micro atomic units, so you have to divide by 1 million. For example Ce-140 is shown as 139905439 which is actually 139.905439 amu's (atomic mass units).
I hope this helps
Bob S

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