# Chemistry

posted by on .

I have a problem that I'm wondering is a "trick" question.

How many nucleons are there in an atom of 235 (atomic mass) over 92 (atomic #) U? According to the periodic table, the atomic mass is 238, not 235. Any comments? Thanks a lot!

• Chemistry - ,

Yes. You must remember that the mass shown on the periodic table is an average of all of the isotopes of an element. U, for example, has isotopes (remember an isotope has the same atomic number but a different number of neutrons--thus, the mass number will be different) of U232, 233,234, 235, 238 and perhaps some others I've forgotten (but you can find them at www.webelements.com). The two MAJOR isotopes of U are U238 (about 99.3%) and U235(about 0.7%). The U. S. learned how to separate the U235 from the U238 in WWII and make the atomic bomb with U235. It will fission but U238 will not.
So the question is not a trick question. The number of nucleons in U235 is 235. You can determine the number of neutrons by 235 total - 92 protons = ?? # neutrons.