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The atomic mass number or atomic mass of an element is the sum of the number of protons and number of neutrons in its nucleus. For example, if an element has p protons and n neutrons in it nucleus, its atomic number is p and its atomic mass is p+n.
Now all atoms of an element do not have the same number of neutrons. This gives rise to several different isotopes of the element. The average atomic mass of an element can therefore be taken as the average mass of each of the different isotopes multiplied by their natural abundance. As a simple example if an element has 5 protons but half the atoms have 5 neutrons and the other half have 6 neutrons, the average atomic mass would be (1/2)*(5+5) + (1/2)*(5+6) = 10.5
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