the idea is ... relative atomic mass = ?(relative abundance x isotopic mass) over all the naturally occuring isotopes
so..
(1).. look on a periodic table to get relative atomic mass
(2).. setup the equation and solve for isotopic mass of the other isotope
(3).. estimate mass # from that isotopic mass.. example.. 70.9247050amu.. probably has mass # = 71
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69.72 = (0.39892 x 70.9247050) + ((1-0.39892) x Z)
Z = 68.92
so we'll guess the other isotope is 69^Ga
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and you can check your answer here
http://en.wikipedia.org/wiki/Galliumand the bottom table on the right clearly shows Ga-69 and Ga-71 are the two naturally occuring isotopes
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This is pretty basic chemistry. I'm surprised that only 2/3 people answering this question got it correct... well.. then again.. maybe I'm not surprised.
This.. "Ga-69 and Ga-71 make up the isotope in the periodic table as Ga-69.723 amu." is nonsense. The naturally occuring isotopes of Ga are Ga-69 and Ga-71. But they don't combine to make "1 isotope as shown on a periodic table". They make a mixture.
If someone were to take a take a sample of 100,000 atoms of naturally occuring Ga and count how many were Ga-69, they would find count 60108. If they counted the number of Ga-71 atoms, they would count 39892. They would never find any Ga-69.723 atoms. The mass of those 100,000 atoms would be 69.723x10^5 amu
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do you have any questions Elena?
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