About Nuclear Masses?

Question

Ok if the definition of an AMU is 1/2 of the mass of a C-12 atom, then why aren't all nuclear masses round even decimals? This shows that no two protons and neutrons are the same size does it not? If they are the same masses, then what accounts for all the decimal parts of the atomic masses? Look on any table that gives nuclear masses in amu's. C-12 has a mass of 12.000000 but that is the only round one. Why?

Answer 1

As far as is known every proton and every electron are identical to each other.

The reason that some elements have decimal values for their atomic masses is that not every atom of a particular element is identical; different isotopes of a particular atome have different numbers of neutrons in them. These neutrons are almost identical to protons in mass. So differences do add up.

The atomic masses are the average of the masses of the various isotopes of an element.

Answer 2

Check your formula. One AMU is 1/12th of C. The mass of atoms includes the mass of the electrons and, since we can not weigh a single atom or a single proton or a single nuetron, we weigh very large numbers of atoms. The exact number of atoms weighed is difficult (impossible) to know exactly. Nothing is 100% pure so there are different kinds of atoms mixed in. All elements have isotopes so not all the atoms of a single element weigh the same. There are lots of sources of error in these measurements. On the plus side, scientists have been making measurements of these things for many years and when improvements in instrumentation and methods come along, the answers are refined. If you looked at atomic weights from 100 years ago, you would find many elements missing (had not been discovered yet) and you would find the atomic weights different.

I hope this helps

Answer 3

well then you have to factor in the electrons surrounding the nucleons of the atom. those are a tiny fraction of the mass of a proton or neutron.

and the atomic masses on the periodic table are averages of all the isotopes of that element found in nature and the proportions they are found in. there could be a helium-3 atom, helium-4, helium whatever, you would have to find the masses of those and then how abundantly they are found on earth. after all of thats done i highly doubt that you would come up with a round number. but i mean look at oxygen, its 15.99. you cant get much closer than that. the .01 off from16 is because of other oxygen isotopes found rarely on earth.