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Why is 1/12th the weight of carbon 12 isotope used to calculate relative atomic mass

Why is 1/12th the weight of carbon 12 isotope used to calculate relative atomic mass

Grade:8

1 Answers

Wajahat Wajid Khan
33 Points
7 years ago
I assume you mean as the definition of an atomic unit. Since protons and neutrons have slightly different weights, and average of the two is used to determine an atomic unit. Carbon-12 is the most common and readily available isotope to have exactly the same number of protons and neutrons, 6 of each, and thus provides a perfect average when divided by the total number of protons and neutrons. (Carbon-12 also has 6 electrons, but the mass of an electron is so small as to be considered negligible).  Dividing by 12 allows even the smallest atom, hydrogen, to have a mass that is a whole number, rather than a fraction.

There are several atoms smaller than carbon-12 with the same number of protons and neutrons, (for example, dueterium, the hydrogen isotope present in heavy water), but they are much less common. By setting the most common isotope of carbon as the standard, it insures that it is readily available to anyone who needs access to it. Hope this helps!

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