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
element

EXAMPLE:
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 EXAMPLE ABOVE:
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?