Question
Thu December 15, 2011 By:

thanks 4 replying to my question about atomic mass... now i understood that why do we call it relative mass, but i still have a query(which i had asked in same question but 4 some reason it wasn't replied). So, my question is, if i want to find atomic mass of say NITROGEN, why should i compare it carbon-12, or as mention in video by mona maam y should i compare it with hydrogen and say that its 14 times heavier .....instead y cant i simply say that nitrogen has 7 protons and 7 neutrons, i.e., 7+7 amu= 14 amu. Basically i want to ask, y do scientist compare it something, y its not simple business of no. of protons+no. of neutrons.(thats what we do in real time,so y definations are confusing ??? )

Expert Reply
Fri December 16, 2011

Standard atomic weight is the average relative atomic mass of an element in the crust ofEarth and its atmosphere. This is what is included in standard periodic tables. Atomic weight is being phased out slowly and being replaced by relative atomic mass. If you will go through the periodic table you will find the atomic mass of nitrogen is not 14, it is 14.0067; of oxygen it is not 8+8= 16 it is rather 15.9994.

 
Chemists used a scale that showed that the natural mixture of oxygen isotopes had an atomic mass 16. Physicists assigned 16 to the atomic mass of the most common oxygen isotope. Problems and inconsistencies arose because oxygen 17 and oxygen 18 are also present in natural oxygen. This created two different tables of atomic mass. A unified scale based on carbon-12 is used to meet the physicists’ need to base the scale on a pure isotope and is numerically close to the chemists’ scale
Mon March 13, 2017

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