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Name:   Art C.
Status:  other
Age:  40s
Location: N/A
Country: N/A
Date: 2000


Question:
I noticed that the atomic weight of Hydrogen was about 1.0079 I assume that this weight comes from the combined weight of one proton, electron and neutron. The question I have is why doesn't helium have twice the atomic weight of hydrogen, since it has 2protons, 2 electrons and 2 neutrons? Are the protons and neutrons basic building blocks like electrons? If so, why is helium 4 times the weight of hydrogen.


Replies:
Art,

The atomic weight of hydrogen (as is the case for all elements) listed on the Periodic Table results from the weighted average of all isotopes of the element. Isotopes are different mass variants of the same element. For example, Hydrogen has three isotopes, protium (one proton and one electron); deuterium (one proton, one neutron, and one electron); and tritium (one proton, two neutrons, and one electron). Notice that all three isotopes have only one proton. It is the proton number that gives the element its identity.

In your question, you compared deuterium (the combined weight of one proton, electron and neutron") to helium. Helium has four isotopes: He-3, He-4, He-5, and He-6. Each differs from the others in numbers of neutrons in the He nucleus. The common form of He is He-4. The other isotopes are rare.

As mentioned above, determination of atomic weights (masses to be more correct) must take into consideration not only the mass of all the individual isotopic variants, but their abundance's as well. Thus, the weights (masses) on the Periodic Table are not the simple sum of proton and neutron counts.

Regards,
ProfHoff



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