isotopic mass

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well "mass" is how much matter there is in something....

the mass in atomic nuclei consists of protons and neutrons...
As you probably know, the number of protons there are in a nucleus determines which element it is..... we chemists don't really care how many neutrons there are, though ;-) so if nuclei have the same number of protons, but different numbers of neutrons, we still say they're the same element - but each species with a different number of neutrons is called an "isotope" of that element.

so, "isotopic mass" is just how much mass there is in an isotope..... in SI units, you'd measure this mass in kilograms -)
But since isotopes are incredddddddddddddibly small, their mass in kg would be like 10 to the minus 12 or sthg like that .....
...so it's more handy to use a "relative" mass scale,
(relative to the mass of Carbon-12 isotope ? )

so we can say "carbon-12" is "12 atomic mass units",
and measure the rest compared to that.

you ask a hell of a lot of questions here, noidea. man, if i was feeling just a touch more irritating, i'd make a crap pun from your username. but i doubt anyone would find it funny... so i won't...

what mr feline there said was very correct and wonderful.

an addendum - relative atomic masses are worked out from the relative abundancies of different isotopes. ie. chlorine (to use the most common example) has two main isotopes - Cl-35 and Cl-37. the (very) approximate abundancies of these are 75% and 25%.. so the relative atomic mass of these is worked out to be approximately 35.5. which explains that weird number you see on the periodic table. because very few elements are only found with all their atoms containing the same number of neutrons.

yeah?

Oh yeah! sorry, I completely forgot about the aspect of taking account of the relative natural abundances of isotopes in nature.

I've always wondered if that part of it isn't a bit dubious -
we're assuming a lot, are we not, if we say that the samples we've studied from the earth's crust are representative of their entire universe?!

I think the reasoning is that particular nuclear synthesis routes (in stars?) for the elements seem to have fairly constant product ratios.

But then again, aren't moon rocks, mars rocks and meterorites easily identifiable by their "isotopic signature"? (ie, they have *different* relative abundances of isotopes from terrestrial rocks)

Mind you, I suppose if you do measure out your compound on the digital scales in the lab to weigh it, the chances are it's come from a chemical supplies catalogue, not the moon, so the relative mass data you use from your periodic table is probably pretty sound ;-)

...

sorry for annoying all my fds, but if you dont ask question here. Then how to use this forum :lol:

opening a book is always a good start too. :P

While I endorse Mitch's idea of opening books whenever possible, go ahead and ask questions noidea; if it REALLY annoys people, they can ignore the questions; there is no obligation to answer or even open every posting.

BTW how do u know feline is a mister?

well you can click on my profile and follow the link to my website...

[quote="feline1"]Oh yeah! sorry, I completely forgot about the aspect of taking account of the relative natural abundances of isotopes in nature.

I've always wondered if that part of it isn't a bit dubious -
we're assuming a lot, are we not, if we say that the samples we've studied from the earth's crust are representative of their entire universe?!

I think the reasoning is that particular nuclear synthesis routes (in stars?) for the elements seem to have fairly constant product ratios.

But then again, aren't moon rocks, mars rocks and meterorites easily identifiable by their "isotopic signature"? (ie, they have *different* relative abundances of isotopes from terrestrial rocks)

Mind you, I suppose if you do measure out your compound on the digital scales in the lab to weigh it, the chances are it's come from a chemical supplies catalogue, not the moon, so the relative mass data you use from your periodic table is probably pretty sound ;-)[/quote]

Heh. I think that last part of your quote there pretty much sums up what I'm thinking. Since all of our chemistry and chemical reactions and whatnot are occuring here on Earth, using the data we've obtained from our terrestrial surroundings seems like a good idea. If a star fifty gajillion light years away has a different isotopic ratio than we do here on Earth, so what? :P

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