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atomic mass unit (AMU or amu)

An atomic mass unit (symbolized AMU or amu) is defined as precisely 1/12 the mass of an atom of carbon-12. The carbon-12 (C-12) atom has six protons and six neutrons in its nucleus.

In imprecise terms, one AMU is the average of the proton rest mass and the neutron rest mass. This is approximately 1.67377 x 10 -27 kilogram (kg), or 1.67377 x 10 -24 gram (g). The mass of an atom in AMU is roughly equal to the sum of the number of protons and neutrons in the nucleus.

The AMU is used to express the relative masses of, and thereby differentiate between, various isotopes of elements. Thus, for example, uranium-235 (U-235) has an AMU of approximately 235, while uranium-238 (U-238) is slightly more massive. The difference results from the fact that U-238, the most abundant naturally occurring isotope of uranium, has three more neutrons than U-235, an isotope that has been used in nuclear reactors and atomic bombs.

Also see our Table of Physical Units and Constants.

This was last updated in January 2011

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Please, I want to know why it was the carbon atom? Why it was not another atom , i.e. why it was not 1/23 from sodium nuclide weight is not it equal to 1/12 from carbon nuclide weight?
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Please I want to know why it was the carbon atom?
why it was not another atom , i.e. why it was not 1/23 from sodium nuclide weight is not it equal to 1/12 from carbon nuclide weight?
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Because, carbon is considered as the unified mass for the measurement!
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How did the scientist of that time know the mass of the carbon atom and they why choice this atom?
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How the scientist of that time know the mass of the carbon atom and they why choice this atomWhy is it that when I try to research on the internet about," how to remove a proton from the nucleus of an atom nothing useful pops up. I just want knowledge not people thinking they know what their talking about. I know that you have to use gamma radiation an that ejecting a particle with gamma particle an changing an element is the way you do this process. But I'm seriously not seeing the scientific research an data to explain how this process works or is engineered. I'm sure that someone can explain to me nuclear and molecular physics for free but how when u work all the time and have kids an can't do your dream. Anyways sorry for rumbling an can someone please help me with finding a way to eject a proton from the nucleus without using a freaking gamma ray gun and hoping for a new element because that's what it feels like all I can find on the web is.
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Why is it that when I try to research on the internet about," how to remove a proton from the nucleus of an atom nothing useful pops up. I just want knowledge not people thinking they know what their talking about. I know that you have to use gamma radiation an that ejecting a particle with gamma particle an changing an element is the way you do this process. But I'm seriously not seeing the scientific research an data to explain how this process works or is engineered. I'm sure that someone can explain to me nuclear and molecular physics for free but how when u work all the time and have kids an can't do your dream. Anyways sorry for rumbling an can someone please help me with finding a way to eject a proton from the nucleus without using a freaking gamma ray gun and hoping for a new element because that's what it feels like all I can find on the web is. A bunch of b.s..
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This is also fine but what the actual definition is of that? And why only the carbon? It can be in other elements or not?
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Carbon is considered as the unified mass for the measurement!
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how we can know the atomic mass unit if there is not given?
eg.if the given is only mass defect, how to find the atomic mass unit(amu) from a given mass defect?
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Since both the proton and neutron have mass values greater than the AMU value it cannot be the average of the two.
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