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Average Atomic Mass

 An element can exist in a number of forms, called isotopes. Isotopes are forms of the same atom that vary in mass as a result of a different number of neutrons.

 There are two naturally occurring isotopes of copper.  One isotope weighs in at 62.93 amu, the other has a mass of 64.94 amu. (atomic mass units)  These two isotopes have different proportions in a natural sample of copper.  The lighter isotope is more common with an abundance of 69.09% of all naturally occurring copper.  What is the percent abundance of the other isotope? Isotopes of Copper

 The remainder of the atoms, 30.91 %, have a mass of 64.94 amu.

 The calculation of the average atomic mass is a WEIGHTED AVERAGE.  To find the AVERAGE ATOMIC MASS of an atom, we take into account all of the isotopes that exist and the percentage of each type.

Average atomic mass = Σ (mass of isotope × relative abundance) Σ = sum or to add them all together

 Since there are two isotopes for copper, we will be adding the contributions of 2 isotopes. (That’s where the Σ sign comes in)  The relative abundance is simply the percentage of the isotope, but in decimal form.  69.09% corresponds to a relative abundance of 0.6909.

Average atomic mass of copper = (62.93 amu × 0.6909) [the mass and abundance of isotope #1] + (64.94 amu × 0.3091) [the mass and abundance of isotope #2] = 63.55

 An AVERAGE atom of copper has a mass of 63.55 amu.  Notice that in this problem, we could predict that the average is closer to the weight of the lighter isotope. Why?  The lighter form of copper is more abundant.

Isotope nameIsotope mass (amu)Relative abundance Silicon – 2827.9892.21% Silicon – 2928.984.70% Silicon – 3029.973.09% Isotope nameIsotope mass (amu)Percent abundance Silver-107106.9050951.86 Silver-109108.90470remainder Isotope nameIsotope mass (amu)percentage Iron – 5453.945.90% Iron – 5655.9391.72% Iron – 5756.942.10% Iron – 5857.930.280%

 Silver = 107.87 amu  Silicon = 28.09 amu  Iron = 55.84 amu

 Read section 1.4 in the text to support what we did in class today.  pg 27 #1  Pg 29 #1-5,9

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