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					FINDING THE ATOMIC
MASS OF AN ELEMENT

by finding the weighted
average of the masses of
 each of the element’s
        isotopes.
              Mass number
   The mass number of an individual atom is
    the sum of protons and neutrons in the
    nucleus.


    Mass number = p + n
   Mass number apply to individual atoms or
    to an element with just one isotope in it.
   Mass number is always a whole number.
 Elements, as found on earth
consist of a mixture of isotopes
        of that element.




 Example: Nearly all of the carbon in a
 sample of pure carbon is carbon-12.
However, the sample also has traces of
  carbon-13 and carbon-14 isotopes.
         Atomic mass




           The atomic mass
         of an element is the
       WEIGHTED AVERAGE
        of the mass numbers
of all of the isotopes in the sample.
What is the atomic number
        of chlorine?




 What is the atomic mass
        of chlorine?
Mass number ≠ atomic mass

Mass number = protons + neutrons

Atomic mass = weighted average of
the mass numbers of all of the isotopes
  of an element present in a sample.
    What is a
weighted average?
               Weighted average
A total of 50 students at Carmel HS took the AP
exam in U. S. History. Here are the results of the
  test. What is the weighted average score?

     Test score        Number of students
           5                      12
           4                      18
           3                      14
           2                       6
           1                       0
A total of 50 students at Carmel HS took the AP exam in
   U. S. History. What is the weighted average score?

Test score    Number of      Multiply        Product
               students

     5           12           5 x 12 =          60

     4           18           4 x 18 =          72
     3           14           3 x 14 =          42
     2           6            2x6=              12
     1           0            1x0=              0

                  Sum =                        186
Weighted average score = 186/50 =             3.72
The Eminemium exercise

 M & M s
The element   Eminemium
    has several isotopes
       • Each color represents an
         isotope of the element. For
         example red is E-20.
       • Count your M&Ms and figure %
         abundance of each isotope.
       • The weighted value for each
         isotope is:
        mass number x % abundance.
The weighted average of the mass numbers of
 each of the isotopes of eminemium is the sum
    of the weighted values for each of the
        isotopes (colors) of eminemium.




This weighted average is the atomic mass of
       that sample of eminemium atoms.
Let’s try the exercise.
      In case you’re curious…
     The M&M website shows the relative
   distribution (% abundance) of it products.




http://us.mms.com/us/about/products/milkchocolate/
Let’s do a calculation for
using the real isotopes of
the element magnesium.
The truth about mass numbers

 Modern chemical instrumentation
 enables us to determine the actual
 masses of each of the isotopes to a
  tremendous degree of accuracy!
Therefore, magnesium-24 has been
  found to have an actual mass of
     23.985 atomic mass units.
   Calculation of the weighted
average of isotopes of magnesium
  Isotope   Actual mass      Relative
              (AMU)       abundance (%)
  Mg-24       23.985          78.99

  Mg-25       24.986          10.00

  Mg-26       25.982          11.01

                            100.00%
  Isotope      Relative    Weighted value
Atomic mass   abundance

23.985        78.99%             18.946

24.986         10.00              2.499

25.982         11.01              2.861

              100.00%     Sum:   24.306
 The weighted average of mass
number of magnesium isotopes
    as found in nature is its
        ATOMIC MASS.


Atomic mass of Mg = 24.306

				
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posted:10/28/2011
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