I’m so confused with these two. I know they are different terms but the periodic table shows atomic mass when to my understanding it should be the atomic weight because it’s taking into account their isotopes
In: Chemistry
The terms are often used interchangeably and arguably context is the only functional differentiation in my experience, but I think the periodic table displays what should be called the atomic weight (the weighted average of the atomic mass of all natural isotopes of that element) and atomic mass is specifically the mass of a single atom.
For example, carbon has an atomic weight of 12.0107 (from memory, might be off a bit) and a carbon-6 atom has an atomic mass of 12.
Source; Masters degree in Chemistry, albeit over a decade ago now
Some do. The *only* thing standardized is the periods & groups, and even those may be presented differently (if you want to “correctly” display the lathanides & actinides). Most tables I have seen have the standard atomic weight listed, not the atomic mass. I have seen some with the atomic weight listed, *and* the atomic mass of the most-common isotope.
The periodic table is mostly used in chemistry where you normally work with large amounts of chemicals in natural isotopic Distributions. For many calculations you want to do with chemicals the value stated in the periodic table is the one you want. Like if have a bunch of sodium chloride, and want to calculate the mass you need of it to get 1 mol (an specific number of molecules), you need the element weight, as all “normal” chemicals will follow these numbers (and even if not the differences are often pretty small).
The difference between the average weight according to natural Isotope distributions and the true mass of the isotopes only matter, when working with single atoms, or isotopic pure chemicals. And if you do that you normally know where to look up the correct masses.
These values periodic tables are just some quick look up tables for rough calculations, that is not a necessary part of periodic tables. Besides if you know your Isotope it’s not so difficult to calculate the true mass (U-238 is approx. 238 u, if you need a more precise number you can look it up in nuclid maps or other reference tables).
The weight listed is the average of all naturally occurring isotopes. For example, Chlorine and Bromine have an almost 50/50 split between their two naturally occurring isotopes, so the weight listed on the periodic table is pretty much in the middle of them.
are you confused because the atomic weight is listed in AMU? atomic weight and mass are measurements of the same thing. one is just an average and the other isnt.
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