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More about isotopes Atomic mass vs average atomic mass or atomic weight

Atomic Mass  The mass of a specific isotope of an element expressed in atomic mass units (amu)  The atomic mass is the total mass of the protons, neutrons and electrons in the atom  Atomic mass = mass of an atom  This is what we refer to in our isotope expression, eg. 

Average Atomic Mass (also called Atomic Weight)  This is the number that you see in the periodic table.  This is the weighted average of the all of the isotopes of Hydrogen 1 H 1.008 1 H 1.008 Atomic Number Symbol of element Atomic Weight

What is a Weighted Average?  The isotopes of an element do not occur with equal frequency or amounts  For example, the relative abundances for the three carbon isotopes are:  carbon-12 98.9%  carbon-13 1.1%,  carbon-14 <001%.  The "average" mass for the atoms of an element is dictated by the most abundant or common isotope  The atomic weights in the periodic table = weighted averages  Tabulated atomic weight value doesn't match any actual atom, but is closer to the weight for the most common isotope  We’ll follow the steps of how to calculate a weighted average next….

Calculating Weighted Average  Carbon-12 atomic mass = 12 amu; abundance = 98.9%  Carbon-13 atomic mass = 13 amu; abundance = 1.1%  Carbon-14 atomic mass = 14 amu; abundance = 0.001% weighted average = ( decimal fraction A) mass A + ( decimal fraction B) mass B Atomic weight = (0.989 * 12) + (0.011 * 13) + (0.00001 * 14) = 12.011 6 C 12.011 6 C 12.011

Average Atomic Mass Worksheet Homework - I will post answers on website Monday. You will have to show ALL of your work.

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