How do you know what isotope has a higher abundance?

Answer 1

You need to know the mass number of the isotope, which is equal to the total number of protons and neutrons in the nucleus. Whichever isotope has the highest mass number is heaviest. It really is just that simple. Let's look at a couple of examples.

If we compare the isotopes carbon-12 and carbon-14, we'll find both have 6 protons in their nuclei. They must if they are carbon. But there are 6 neutrons in the nucleus of C-12, and 8 neutrons in the nucleus of C-14. The C-14 isotope has a greater mass. When we look at uranium-238 and uranium-235, the U-238 has more mass. Both isotopes have the characteristic 92 protons, but there are 146 neutrons in the U-238, and there are only 143 neutrons in the U-235. If you've figured out that the specific number associated with a given isotope is the number of protons plus the number of neutrons in its nucleus, you are absolutely correct. That's where the number comes from.

See the Related Questions for more information.

Answer 2

You can figure it out by comparing the two elements and there average Atomic Mass to the one on the Periodic Table, and which ever is closest to the isotope on the periodic table is the most abundant.

Answer 3

Experimentally, I would use mass spectrometry to sort a sample of known chemical formula into fractions of mass and then find the most abundant isotopic peak. Best if you can fraction the molecule so greatly that you end up with an atomised element, then finding it's isotopes in the spectra would be far easier.

But this is already done for us, by now. So instead, I go to wikipedia :P.

If you type an element name into wikipedia, go to it's page and look down the right-hand side. It's isotopes, their half-lives and their abundances are given.

Answer 4

Look at the average molar mass; the isotope with the mass closest to that value will most likely be the most abundant.

Answer 5

The nature and abundance of isotopes for a chemical element is determined by mass spectrometry.