SCH 3U1
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Isotopes and Average Atomic Mass
Atomic mass units are a relative measure used to express the masses of
atoms and molecules.
All atomic masses relate to the carbon-12 atom
Therefore 1u = 1/12 of the mass of a carbon atom
Isotopic abundance: the relative amount in which each isotope is present in
an element.
The average atomic mass (mass number) of carbon on the periodic table
is 12.01u, why? ___________________________________
There are three isotopes of carbon, they are carbon-12, carbon-13 and
carbon-14
Each is present in differing amounts
Carbon-12 makes up 98.9%
Carbon-13 makes up 1.1%
Carbon-14 makes up 1X10-10 %
We calculate average atomic mass using weighted averages.
Example 1:
Lithium exists as lithium-7 and lithium-6
Lithium-7= 7.015u and accounts for 92.58%
Lithium-6= 6.015u and accounts for 7.42%
7.015 u (0.9258) + 6.015 u (0.0742) = _____