On 19/04/2011 00:07, Tom wrote:
At 12:56 PM 4/18/2011, you wrote:
> I have an idea that the definition I came across specifies Carbon-12.
> Since the atomic masses are not exact integer rations of each other (in
> other words, an oxygen-16 atom is not exactly 4/3 of the mass of a
> carbon-12 atom), I think you do have to specify the element you are using
> here.
Trying to remember my Chemistry A level from 25 years ago....
I believe that an Atomic Mass Unit (AMU), is defined as 1/12 of the mass
of a Carbon 12 neucleus. The reason for choosing C-12 is that it has an
equal number of protons and neutrons, so the slight difference in mass
Yes, but it's hardly unique in that. So does oxygen 16 )8 of each),
helium 4 (2 of each), etc.
Hwoever, a carbon 12 atom does not have _exactly_ 3 times the mass of a
helium 4 atom. The difference is minute, but when you're defining a unit,
it matters.
is averaged out. In the past Hydrogen was used but
since (the most
Oxygen 16 was also used at one time (a different time :-))
common isotope) of Hydrogen only contains 1 proton,
this lead to
descrepancies for any nucleus with neutrons in.
Actually you could pic any atom you liked as the basis of the definiton.
The thing is that the exact value of Avogadro's number depends on which
atom you use. So you have to specify one.
-tony