Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: bgreeson on April 27, 2010, 09:18:59 PM

Title: Question about the amu scale
Post by: bgreeson on April 27, 2010, 09:18:59 PM

So I understood, initially, that the scale of amu(atomic mass unit) is based upon 1/12 the mass of carbon-12. This made sense to me until another section in my textbook listed the mass for Electrons, protons, and neutrons. So we know that carbon-12 has 6 protons, and 6 neutrons. Now we also know that the mass of a proton and a neutron are both greater than 1, this is greater than 12 without even factoring in the mass of electrons(negligible). Here is my math below.

6*Mass of Proton + 6*Mass of neutron

6*1.0072764669amu + 6*1.0086649156amu > 12 amu

So without even factoring in the electrons we are over 12amu for one atom of carbon-12. To me this says one of these principles is violating the definition of the other. No one I ask can explain why this happens, and what the real rule is. Can anyone explain why these numbers don't agree? My book doesn't see the need to explain an obvious contradiction. Thanks.

Title: Re: Question about the amu scale
Post by: gregdwulet on April 28, 2010, 01:29:49 AM

Interesting question. I don't know the answer, but I suspect it has to do with quantum mechanics; I do know that the collective weight of protons and neutrons is dependent on the distance between the actual particles, due to the nature of the strong nuclear force that holds them together. Since the protons and neutrons are separated by a very small amount of space in the nucleus, the nucleus as a whole weighs more than just the sum of all of the protons and neutrons added together.

This is just a guess, does anyone know the real answer?

Title: Re: Question about the amu scale
Post by: Borek on April 28, 2010, 03:39:08 AM

http://en.wikipedia.org/wiki/Mass_deficit

Title: Re: Question about the amu scale
Post by: bgreeson on April 28, 2010, 10:06:26 AM

"Definition: The difference between the unbound system calculated mass and experimentally measured mass of nucleus is called mass defect. It is denoted by Δm. It can be calculated as follows:

    Mass defect = (unbound system calculated mass) - (measured mass of nucleus)

        i.e, (sum of masses of protons and neutrons) - (measured mass of nucleus)"
-Wikipedia

So does the mass of 12 amu for carbon-12 refer to the "measured mass of nucleus"? And does this exclude binding energy? While the number > 12 is the sum of masses of protons and neutrons?

Assuming I have a rough understanding of why this descrepancy occurs now, I am still not sure at which time, which measure is appropriate, or which one is more "official".

Title: Re: Question about the amu scale
Post by: bgreeson on April 30, 2010, 08:38:44 PM

Can anyone explain this ^^

Title: Re: Question about the amu scale
Post by: bgreeson on May 06, 2010, 08:12:57 PM

Does anyone know this?