Re: how is it that the Atomic Mass Unit is less than either the
- From: Gerry Quinn <gerryq@xxxxxxxxx>
- Date: Tue, 19 Sep 2006 15:12:13 +0000 (UTC)
In article <C1322FB8.1A7E%rbj@xxxxxxxxxxxxxxxxxxxx>,
rbj@xxxxxxxxxxxxxxxxxxxx says...
the amu is defined to be 1/12 the mass of a carbon-12 atom, i presume
complete with 6 protons, 6 neutrons, and 6 electrons.
even if the electrons weighed nothing, each proton and neutron is more
than the amu so it must add to more than 12 amu. how can this be?? i
know that this is a definition, but it seems to me that the definition
does not square with CODATA values of the most massive components of a
carbon-12 atom and the conservation of mass principle. how does
sticking these 6 neutrons and 6 protons together, toss in 6 electrons
and maybe some binding energy/c^2 add up to less than the total? i
don't get it.
i had always assumed that the amu was somewhere nearly halfway between
the proton and neutron rest mass, but it's less than either. how can
that be?
can someone make sense of this?
In a nutshell: binding energy is *negative*. It's minus the energy
that is released when protons, neutrons, and electrons combine (bind)
to form an atom; or put another way, it's minus the energy you'd need
to add back if you wanted to split them up again.
6P + 6N + 6e = Carbon + "released energy".
The binding energy is minus the "released energy" in the above, so:
6P + 6N + 6e + binding energy = Carbon
- Gerry Quinn
.
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- From: robert bristow-johnson
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