Chemistry - Isotopes and Weighted Average Atomic Mass HWK 2-3
Name :___________________
Period: _______
If the atomic mass is the sum of protons and neutrons in an atom, why are the atomic
masses on the periodic table not all whole numbers? Because the atoms of elements come
in a variety of isotopes, meaning they are made up of atoms with the same number of
protons (atomic number) but different number of neutrons in the nucleus. The atomic
number on the periodic table is a “weighted average” of the naturally occurring isotopes of
that element. The weighted average atomic mass (sum of neutrons and protons) takes
into account the fact that there are different amounts of each type of isotope
(abundance) in a naturally occurring sample of any given element.
The weighted average atomic mass is calculated by multiplying the decimal equivalent of
each isotope times its mass and adding up all the results for all the naturally occurring
isotopes.
Example:
A sample of Cesium, Cs, has the following % abundance:
Cs-132 = 20.0%; Cs – 133 = 75.3%; Cs – 134 = 4.7%
Weighted average atomic mass = (0. 0.20 x 132) + (0.753 x 133) + (0.047x 134) = 132.9
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Determine the average atomic mass for the following mixtures of isotopes:
1.) Au – 197 = 50%, Au 198 = 50%
2.) Fe – 55 = 15%, Fe – 56 = 85%
3.) N-14 = 95%, N-15 = 3%, N-16 = 2%
4.) C-12 = 98%, C-14 = 2%