ISOTOPES AND AVERAGE ATOMIC MASS
ISOTOPES: atoms of the same element that have different masses
Q: What particles make up
the mass of an atom?
A: protons (p+) and neutrons (no)
So, isotopes of the same element have the
same number of _______________, but have different numbers of _______________.
MASS NUMBER: # of p+ plus # of no
Q: Why is the mass given
on the Periodic Table a decimal number (not a whole #)?
A: isotopes of elements occur in nature a certain % of the time. Decimal # on the P.T.
(called the "AVERAGE ATOMIC MASS") is the weighted average of all isotopes of that
Chlorine exists in nature as two different isotopes. Chlorine - 35 has a natural abundance of 75.8 %. Chlorine - 37 has a natural abundance of 24.2 %. What is the average atomic mass of chlorine? (On these problems, you MUST show your work in order to receive any credit.)
35Cl: 35 amu x 0.758 = 26.53
37Cl: 37 amu x 0.242 = 8.954
26.53 + 8.954 = 35.484 ==> (rounded to 3 SFs) = 35.5 amu
TRY THIS ONE ON YOUR OWN FOLLOWING THE
An imaginary element with the symbol Lb has 3 naturally-occurring isotopes. 40Lb has a percent abundance of 41.0 %. 42Lb has a percent abundance of 48.3 %. 43Lb has a percent abundance of 10.7 %. What is the average atomic mass of element Lb?