Why do elements have mass numbers?

Question

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:

The mass number of an element is the mass number of the majority isotope.

Would appreciate anyone willing to clear this out for me. Thanks!

a row in the chart says
element: Na atomic number: 11 mass number(A): 23 Atomic mass(u): 22.99 — 12345bird, Apr 19 '20 at 19:02
What does it say for an element having more than one isotope? (Cl will do.) — Ivan Neretin, Apr 19 '20 at 19:04
it says that we take the weighted average of the isotopes (i think this is the atomic mass). So does an element really have no mass number, but it has atomic mass instead? — 12345bird, Apr 19 '20 at 19:22
By the way, they did not say anything about elements having mass numbers. The only place where they hinted at it is when they wrote a table with elements and mass numbers and other stuff. — 12345bird, Apr 19 '20 at 19:24
When in doubt look at IUPAC Gold Book - mass number: Total number of heavy particles (protons and neutrons jointly called nucleons) in the atomic nucleus . Also called nucleon number . Symbol m in mass spectrometry . — MaxW, Apr 19 '20 at 19:41

Karl · Answer 1 · 2020-04-20T05:02:03.700

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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.

edited Apr 20 '20 at 05:02

answered Apr 19 '20 at 20:09

Karl

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In the first paragraph "rather" is the keyword. The isotopic variation is why atomic masses for many elements have limited precision. // Don't ever remember that "The iridium that came on earth with the impact that killed the dinosaurs has a distinctly different composition than all other iridium deposits on earth..." To my knowledge iridium was a marker because it occurred in higher concentrations in particular geological layers. // Atomic weights are known to much better precision than the weighted mass numbers (where weight is isotopic abundance). – MaxW Apr 19 '20 at 21:30
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Atomic weights might be different on Mars because of isotopic variation, but the mass number of an isotope wouldn't vary. – MaxW Apr 19 '20 at 21:30
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@MaxW There is a singular iridium concentration in sediments from that time, and it has a different isotopic composition, which more or less proves that it is extraterrestic. – Karl Apr 20 '20 at 05:07
Super Fantastic!!! Learned something new. I poked around a bit and you are right the unearthly isotopic composition was one of the pieces of evidence for the Alvarez hypothesis. – MaxW Apr 20 '20 at 05:24

Why do elements have mass numbers?

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