Re: ²²Na
- From: The Ghost In The Machine <ewill@xxxxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 18 May 2007 20:54:10 -0700
In sci.physics, Srinidhi Venkatesh
<srinidhiv123@xxxxxxxxx>
wrote
on 16 May 2007 11:51:27 -0700
<1179341487.842196.149040@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>:
What is ²²Na ? What does it mean when the 22 comes as a superscript
before Sodium? Could you please explain it to me? I have a rough idea,
but my friend has no clue
To answer this question properly requires delving into
the world of isotopes.
As you're probably aware, Na has atomic number 11 --
this means 11 protons in the nucleus and 11 electrons
surrounding it, unless one wants to discuss ionization.
("Orbit" to me evokes the Bohr model, which is quite
outdated, though many here will probably use it; I'm
not sure if I should, or not.) A nucleus with only 11
protons is quite unstable, it turns out; most nuclei
more complicated than hydrogen will have some neutrons,
and for the smaller atomic number atoms the number of
neutrons is roughly the same as the number of protons,
though it doesn't have to be.
The 22 in your notation simply is the total number of
nucleons (protons + neutrons) in the nucleus. Note that
this is not quite the same as atomic weight, which in the
case of pure Na-23 [*] is actually 22.98976928(2).
Only C-12 [+] has them exactly equal -- and that's because
we've defined it that way.
In the case of sodium, Na-23 is the only stable isotope;
Na-22 is a radioisotope with a half-life of 2.605 years,
decaying via electron capture to Ne-22, according to
http://www.webelements.com/webelements/elements/text/Na/isot.html
http://www.webelements.com/webelements/elements/text/Na/radio.html
I'm frankly not sure where one might study the theory of nuclei;
presumably the more sophisticated types are still working out why
Na-23 is stable but Na-22 is not, and why U-235 can fission but
U-238 captures the nucleus.
[*] the notation Na-23 is suggested by U-235, and is an alternate
to prefixed superscripts, presumably.
[+] note that naturally occuring carbon's atomic weight is 12.0107(8);
that's because C-13 is also stable.
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