Atomic Mass Units: Discrepancy Explained?

In summary, the definition of an atomic mass unit (amu) is the mass of a carbon-12 atom, which is exactly 12 amu. However, the mass of a proton and neutron is slightly different, with a proton being about 1.007 amu and a neutron being about 1.009 amu. When adding up the masses of 6 protons and 6 neutrons for a carbon-12 atom, the total mass is slightly more than 12 amu. This discrepancy is due to the "binding energy" of 12C, which is affected by relativistic effects. This means that while an isolated proton may have a mass of 1.007 amu, when it is part of an
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Been many years since I took general and org chem and am currently reading general chem textbook as refresher. Came across defintion of amu (atomic mass unit). Text says that the mass of the carbon-12 atom is defined to be exactly 12 amu. However, a couple of pages over, I read that the mass of the proton is about 1.007 amu, and the mass of the neutron is about 1.009 amu. Add up the masses of 6 protons and 6 neutrons for the carbon-12 atom, and you come up with more than 12 amu. I can't resolve this apparent discrepancy. I've consulted other texts, etc. Does some type of relativistic effect bear on this situation? In other words, does, for example, an isolated proton have a mass of 1.007 amu, but then when part of an atom have a slightly different mass because of some relativistic effect? HELP!
 
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The mass difference is the "binding energy" for 12C.
 
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The discrepancy between the mass of an individual proton or neutron and the mass of the same particle within an atom can be explained by the concept of binding energy. In an atom, the protons and neutrons are held together by strong nuclear forces, which require energy to overcome. This binding energy reduces the overall mass of the atom, making it slightly less than the sum of the individual masses of its constituent particles.

This phenomenon is known as mass defect and is described by Einstein's famous equation E=mc^2. The binding energy is converted into mass, resulting in a decrease in the overall mass of the atom.

In the case of carbon-12, the mass defect is relatively small, but it becomes more significant for larger atoms with more protons and neutrons. This is why the mass of an individual proton or neutron is not exactly the same as the mass of the same particle within an atom.

So, the explanation for the discrepancy between the mass of an individual proton or neutron and the mass of the same particle within an atom is due to the binding energy and its conversion to mass. This is a fundamental concept in nuclear chemistry and is important to understand in order to fully grasp the concept of atomic mass units.
 

1. What are atomic mass units (AMU)?

Atomic mass units (AMU) are a unit of measurement used to express the mass of an atom or molecule. It is equal to 1/12th of the mass of a carbon-12 atom and is approximately equal to the mass of a proton or neutron.

2. Why is there a discrepancy in atomic mass units?

The discrepancy in atomic mass units is due to the fact that the mass of an atom or molecule is not just the sum of the masses of its individual particles (protons, neutrons, and electrons). This is because a small amount of mass is converted into energy during the formation of the atom or molecule, as described by Einstein's famous equation E=mc².

3. How is the atomic mass unit calculated?

The atomic mass unit is calculated by taking the mass of a carbon-12 atom (12.0000 AMU) as the standard and comparing it to the mass of the atom or molecule in question. This is done by using a mass spectrometer, which measures the mass-to-charge ratio of atoms or molecules.

4. Is the discrepancy in atomic mass units significant?

The discrepancy in atomic mass units is very small and is usually only significant in very precise scientific measurements. In most cases, the difference in atomic mass units does not affect the properties or behavior of atoms or molecules.

5. How does the discrepancy in atomic mass units affect chemistry and physics?

The discrepancy in atomic mass units does not significantly affect chemistry and physics. However, it is an important concept to understand in nuclear physics and in the field of mass spectrometry, where very precise measurements are needed.

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