# & Average Atomic Mass  Atoms with the same number of protons (they are the same element) but different number of neutrons.

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& Average Atomic Mass

 Atoms with the same number of protons (they are the same element) but different number of neutrons.

 For some elements, there can be more than one possible mass number.  Why? The neutrons are present to stabilize the protons in the nucleus so that they don’t repel (bounce off each other) too much and keep the atom stable.  Different numbers of neutrons can be effective for making a separation.  Example: Imagine an atom with 3 protons:

 If all isotopes have an atomic mass that’s a whole number, why are all the atomic masses on the periodic table decimals?  There are many isotopes for each element, and the atomic mass given on the periodic table is a weighted average of all their isotope masses.

 Dependent upon both mass and the relative abundance of each of the elements isotopes.  Once you have the isotopic abundances, you can find the average atomic mass of the elements using the following equation:

 Naturally occurring copper exists with the following abundances:  69.17% is Cu-63 w/ atomic mass 62.93 amu  30.83% is Cu-65 w/ atomic mass 64.93  (.6917)x(62.93) + (.3083)x(64.93)= 63.55 amu

 3 Isotopes of Ar occur in nature  0.337% as Ar-36, 35.97 amu  0.063% Ar-38, 37.96 amu  99.6% Ar-40, 39.96 amu  Calculate the Average Atomic Mass

 (.00337)x(35.97) + (.00063)x(37.96) + (.996)x(39.96)= 39.95amu

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