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Isotope abundances
The isotopes of an element do not occur with equal frequency.
The relative abundance depends on the relative stability of the isotope.
The isotopes contribute to the average atomic mass based on their
abundance.
The atomic weights in the periodic table are weighted averages.
This means the tabulated value doesn't match any actual atom, but is closer to
the most common isotope.
Average weight =
% First isotope abundance x its mass + %Second isotope abundance x its massWhat is the average atomic mass for thallium, Tl, if there are two isotopes with the
following masses and abundances? (Tl-203 (203Tl) has a mass of 203.059 amu
with an abundance of 29.52 %, Tl-205 (205Tl) has a mass of 205.059 amu with an
abundance of 70.48 %)
Step 1: Convert percents to decimals 29.52 % to 0.2952 and 70.48 % to 0.7048
Step 2: Average weight = 0.2952 x ( 203.059 amu) + 0.7048 x ( 205.059 amu)
204.466 amu rounded off to 204.5 amu with 4 significant
.