AMU

Anyone know exactly where the numbers came from originally to define the Atomic Mass Units?

Mendelev knew them in 1869

Jons Jakob Berzelius ran experiments and using his experimental results he determined the atomic weights of nearly all the elements then known. They were whole numbers. If you look in the literature they report the value for Carbon as 12 and the value for the Carbon 13 isotope as 13.003 It is those two values that they multiply by the percent abundance and then use to calculate the average mean value of 12.0107 for Carbon. But where did the .003 come from for Carbon 13?

Who put together those fractional components and how did they arrive at them?

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I'm pretty sure they "set" carbon as 12 back in the day before they knew about isotopes. It was the "standard" which all other AMUs were based, hence why it is a whole number, and Hydrogen is like 1.00XXX instead of exactly 1.

When the knowledge of isotopes came along, I'm assuming they just added the mass of an additional neutron which approximately equals 1.003 AMU to Carbon - 12 to get Carbon 13.003.

Don't forget that when protons and neutrons combine in a nucleus,
the overall mass changes from what their original separate masses would've been.
Thus, although we may have a figure for the mass of an isolated neutron, you can't just add this one to other isotopic masses to find a new isotope mass value.

History of the birth of Earth...

When planet Earth was formed from a big cloud of Hydrogen, energy-emitting nuclear fusion started. This fused the elements 1 to 26, which are now known as Hydrogen to Iron.

As fusion ended, the Iron has lost all its energy in fusing. Therefore it's the most lightweight element in comparison to the number of nucleons inside its nucleus.

Then the Earth started a change. Stuff was penetrated by high energy and started further fusing. This time elements 27 through 92 were fused. Each element further away from Iron gets more energy, and therefore heavier in comparison to the number of nucleons.

Finally after milennia, something like Technetium and Promethium decayed, Fission started from Uranium and something like Plutonium is naturally fused---This is the modern planet Earth.

Part of the Answer

Dalton put together the first AMU's by indirect measurement. His chief flaw was in his insistence that in general, molecules were formed by the paring of a single atom of one element with a single atom of another.

It was 6 weeks after Volta's experiments in 1800 that Nicholson and Carlisle first decomposed water by electrolysis and proved that water was H2O and not HO like Dalton had argued. As it turned out, there was 32x the oxygen, by weight, as there was hydrogen so they divided it down and ended up saying that if the weight of hydrogen is set at 1, then, the atomic weight of oxygen must be 16 and not 8 as Dalton had proposed.

After Gay-Lussac announced the Law of Combining Volumes in 1908 someone performed a similar experiment on Amonia and concluded that it's formula was NH3 and Nitrogen's weight was 14.

It appears that Berzelius, the Swedish Chemist, about 1807, was the first one to begin using the fractional component with the AMU's. He was the first one to use numbers that were not whole numbers.

So, where did our modern numbers come from?

http://www.infoplease.com/ce6/sci/A0805234.html

Atomic masses are usually determined by mass spectrography. Here is an outline of "how they directly" measure the mass.
http://www.infoplease.com/ce6/sci/A0832154.html

WebElements: the periodic table on the WWW [http://www.webelements.com/]

Copyright 1993-20010 Mark Winter [The University of Sheffield and WebElements Ltd, UK]. All rights reserved.