From: Ronald Wong (ronwong@inreach.com)
Date: Fri Oct 20 2000 - 01:49:09 PDT
Message-Id: <l03102802b615a2e736bb@[209.209.18.66]> Date: Fri, 20 Oct 2000 02:49:09 -0600 From: Ronald Wong <ronwong@inreach.com> Subject: Re: mass of C-12
Camilla Lau said:
>I have a question about the masses of protons, neutrons, and the carbon-12
>atom. In my texts, it says the masses of proton and neutrons are just over
>1 amu. How does this work, if 1 amu is defined as 1/12 the mass of
>carbon-12? Thanks!
Camilla:
Today, the physical - as opposed to chemical - scale of atomic masses is
based on the most common form of carbon, C12. In the past it was O16.
Carbon-12 is assigned a physical mass of exactly 12 amu.
Physicists determine the mass of an atom using a mass spectrometer. Like an
optical spectrometer, it has to be calibrated. They do this using
carbon-12. When carbon-12 passes through the mass spectrometer, it lands at
a particular location on a scale in the spectrometer. This location
corresponds to 12 amu by definition. Based on the design of the
spectrometer every other location on the scale can now be assigned an amu
value.
When hydrogen passes through it lands at a spot corresponding to 1.00783 amu.
1 amu is 1/12 of 12 amu. The experimental evidence shows that the atomic
mass of hydrogen isn't 1 amu.
Oxygen-16 lands at a spot corresponding to 15.99491 amu - NOT 16 amu, etc.
A neutron has an atomic mass of 1.008665 amu (it's determination is not
made with a mass spectrometer since it is electricallly neutral), a proton,
1.007276 amu, and an electron, 0.000549 amu.
>Furthermore, the masses of other elements are less than their mass number.
Some are less than their mass number(O-16), some are more than their mass
number (B-10), and one (C-12) is equal to its mass number.
>Does some of the mass go into the energy of the atom?
Subtract the mass of the 6 orbiting electrons from the mass of an atom of
C-12 and you have the mass of it's nucleus:
Mass of nucleus = 12.000000-6(0.000549) = 11.996706 amu
The nucleus consists of 6 protons and 6 neutrons. What is the mass of 6
protons and 6 neutrons?
Sum of 6P +6N = 6(1.007276) + 6(1.008665) = 12.095646 amu
The whole is LESS than the sum of its parts!
The difference (0.09894 amu) when converted into units of energy (remember
that mass IS energy) represents the "binding energy". This is the energy
needed to hold the nucleus together (sort of like potential energy). If
you want to break up the nucleus of C-12, you'd need the energy equivalent
of 0.09894 amu).
There are 931.5 Mev/amu. So binding energy = (0.09894 amu)(931.5 Mev/amu)
= 92.16 Mev = 1.476X10^-11 J
92.16 Mev is what is keeping the nucleus together.
The answer to you final question is yes.
92.16 Mev will also be all that it takes to break up the nucleus.
Cheers - ron
This archive was generated by hypermail 2b29 : Tue Oct 16 2001 - 12:21:58 PDT