Re: how is it that the Atomic Mass Unit is less than either the
- From: helbig@xxxxxxxxxxxxxxxxxxxxxxxx (Phillip Helbig---remove CLOTHES to reply)
- Date: Wed, 20 Sep 2006 09:29:41 +0000 (UTC)
In article <C1322FB8.1A7E%rbj@xxxxxxxxxxxxxxxxxxxx>, robert
bristow-johnson <rbj@xxxxxxxxxxxxxxxxxxxx> writes:
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.
The binding energy is negative. Thus, if we put 6 protons and 6
neutrons together, energy is released. As a result, the mass of a
carbon nucleus is less than the sum of its parts. The resulting energy
is radiated away. This is how the sun produces energy, for example. In
the sun, through intermediate steps, hydrogen is converted into heavier
elements such as helium, carbon, oxygen, neon etc.
The process (fusion) stops at iron-56. After that, rather than having
energy released when the mass of the nucleus increases, one has to add
energy. Thus, fusion doesn't produce anything heavier. However, if
there is an external (to the nucleus being transformed) source of
energy, such as in a supernova explosion, then this energy can go into
making heavier elements, which indeed is how they are formed. This
energy, in turn, can be released by decreasing the mass of the nucleus.
This happens in radioactive decay and in fission.
Note that there is no conservation of mass. (This is an approximate
law, codified by Lavoisier, which holds for low-energy transformations,
such as chemistry (one of the many fields in which Lavoisier was
active). The general law is the conservation of energy, taken to
include mass via E = mc².
.
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- From: robert bristow-johnson
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