Question

While searching for various atomic mass units, scientists initially took 1/16 of the mass

of an atom of naturally occurring oxygen as the unit. State two reasons. (atomic masses

O=16, C=12)

Answer 1

Answer : 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!

Answer 2

Answer:

Explanation:

First of all, relative atomic mass, means that what is the atomic weight of any atom compared to atomic weight of carbon atom. Or we can say how many times any atom is heavier then 1/12 mass of carbon.

Now, question arrises that why only carbon?

=> because atomic weight of carbon is 12 and also, no. of proton and neutron were also 6 each which sums up to 12. So it was concluded that carbon will be the best atom for comparison of atomic masses of other elements' atom as it had exactly 12 atomic mass. Surely carbon, has an isotope with atomic mass 14. So you may wonder then what's the point if the average atomic mass of carbon is not exactly 12?

Well, the thing is that the other isotope of carbon is not that common in the atmosphere or hydrosphere or troposphere or etc. It's found in tree barks and other objects which are really old enough that too, in very small amounts. C-14 is radioactive and hence is also useful for calculating the age of a material (carbon dating). And also, may I remind you that carbon 12 is used to calculate relative atomic mass. The average atomic mass of carbon isn't considered for calculating it.