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Home Q & A Board Science-Related Homework Help Chemistry (Moderator: Laser_3)
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fire117 fire117
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11 years ago
the atomic mass of carbon is 12 and that of hydrogen is 1. why ? moreover the mass of hydrogen is not exactly 1 its 1.099. why?
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wrote...
#1
11 years ago
1 u (atomic mass unit) is exactly 1/12 the mass of an carbon atom (12C6 - an atom with 6 proton and 6 neutron)
so, it is clear that a carbon atom will have a mass of 12u

But a proton and a neutron doesn't have the same weight, so 1u is not exactly the mass of a neutron or a proton

A hydrogen has only 1 proton, so it mass is about 1u, but since proton is a little bit heavier, it is 1.099u
wrote...
#2
11 years ago
The atomic mass of an element is the average protons + neutrons of all of the isotopes of that element.  The mass of hydrogen is slightly higher than 1 because there are three isotopes (protium, deuterium, and tritium).  The atomic mass of carbon is 12 because the most common isotope of carbon is C12.  That is 6 protons and 6 neutrons.
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