Why do elements have mass numbers? [duplicate]








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[Mass number, (relative) atomic mass, average mass](/questions/38903/mass-number-relative-atomic-mass-average-mass)

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I don't really understand why an element has a mass number, I thought that an element has several isotopes, each with a different mass. Is my understanding correct: 



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> The mass number of an element is the mass number of the majority isotope.
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Would appreciate anyone willing to clear this out for me. Thanks!




The matter on earth has been mixed rather thoroughly through time, expecially in the beginning 4.5 billion years ago. That means that all elements have a rather (some more, some less) constant isotopic mixture across the earth crust, and so you can assign an atomic weight also to each nonmonoisotopic element.


Exceptions prove the rule, like C-14, deuterium, etc. The iridium that came on earth with the impact that killed the dinosaurs has a distictly different composition than all other iridium deposits on earth, as does chromium from the same sediments.


Of course on mars, the numbers would be slightly different.


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