Answers to Class Questions from 9/15/03


Q: Are there four isotopes for each element or does it differ?  How do we know how many isotopes of an element exist in nature - is there a chart?

Answer:  Each element has a unique number of naturally occuring isotopes.  Some elements have only 1 isotope (Al) while others have many (Mo has 7!).  You can find published tables with the number of isotopes for each element, their natural abundance (%) and their masses.  For some interactive fun, check out the periodic table with isotope data link at:
http://www.webelements.com/webelements/elements/text/periodic-table/isot.html


Q: Can their be isotopes of compounds?

Answer:  Compounds cannot have isotopes; however, since atoms of elements make up a given compound, there can be isotopes of a given element in any compound.  For example, if I have a protein (like COX-2), there might be more than 1,000 carbon atoms in that compound.  Based on the carbon isotope abundances, I would expect out of 1,000 carbon atoms,  989 are carbon-12, and 11 are carbon-13.  


Q: How were isotopes discovered?

Answer:  Many of the isotopes of a given element were discovered when people observed that some atoms of an element were radioactive whereas other atoms were not.  Other isotopes were discovered as a by-product of nuclear chemistry.  Still others were discovered when people built an instrument that could measure the exact mass of a chemical (i.e., a mass spectrometer) and they were able to observe an isotopic pattern to a chemical's mass.  


Q: How do isotopes get more or fewer neutrons?

Answer:  Isotopes are formed via a series of nuclear chemistry reactions that occur naturally or that are designed by human understanding to occur.  The process can release or consume lots of energy in order to achieve products.  We will not study nuclear chemistry, but if you are interested, you can find a substantial amount of information.


Q: How do you calculate the percentage of isotopes in an element?

Answer:  You have to do a little algebra here as well as have a periodic table and exact masses for the isotopes at hand.  I will attempt an example here for you to consider.    Lithium has 2 naturally occuring isotopes:  6Li 6.0151 amu, 7Li 7.0160 amu. The mass given in the periodic table is 6.941 amu.  We want to find what fraction of Li occurs as the 6Li and 7Li isotopes.  Thus, we know the following:

(Fraction of 6Li) + (Fraction of 7Li)  =  1.00

6.941 amu  =  (Fraction of 6Li)(6.0151 amu) + (Fraction of 7Li)(7.0160 amu)

rearrange the first equation and substitute it into the second

(Fraction of 7Li)  =  1.00 - (Fraction of 6Li)

6.941 amu  =  (Fraction of 6Li)(6.0151 amu) + [1.00 - (Fraction of 6Li)](7.0160 amu)

multiply/expand....

6.941 amu  =  (Fraction of 6Li)(6.0151 amu) + 7.0160 amu - (Fraction of 6Li)(7.0160 amu)

collect like terms

6.941 amu - 7.0160 amu  =  (Fraction of 6Li)(6.0151 amu) - (Fraction of 6Li)(7.0160 amu)

-0.0750 amu = -1.0009 amu(Fraction of 6Li)

0.0749 = (Fraction of 6Li)      note: this is 7.49%

now calculate the Fraction of

(Fraction of 7Li)  =  1.00 - (Fraction of 6Li)

(Fraction of 7Li)  =  1.00 - 0.0749  = 0.9251  note: this is 92.51%




Q: How does amu relate to carbon-12?

Answer:  An atomic mass unit is defined by an atom of carbon-12.  The actual value of an amu is 1/12 the mass of a carbon-12 atom.  The reasons for picking this as the basis for definition of amu are varied.  The biggest reasons relate to the high natural abundance of carbon-12 (~99%) and the high occurence of carbon in the chemistry related to living organisms as we currently know them.  


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