Question

Atomic Mass Unit (u): A unit of mass, abbreviated u, that is exactly 1/12 the mass of a carbon-12 atom. 1 u equals 1.66x 10-24 grams.

Answer 1

why did they take carbon as reference and not hydrogen ?

Answer 2

My chemistry teacher has told that hydrogen was once the reference point. I speculate that the change may have been motivated by hydrogen's chemical instability or by its difficulty to obtain in a pure form compared to carbon.

Answer 3

Because if they take carbon then they can specify atomic mass of many elements in terms of its whole number multiples which would not be possible if they would choose hydrogen instead! Remember the atomic mass of hydrogen is 1.008 amu not 1 amu

Answer 4

oh why is that so, hydrogen has 1 proton 1 neutron, ?
carbon has 6 protons, 6 neutrons,

so hydrogen must have exactly 1/6th of the mass right

Answer 5

sorry i just started wid this module and getting all mixed up

Answer 6

The most common isotope of hydrogen contains 1 proton, no neutrons. The other two, deuterium and tritium, contain 1 and 2 neutrons respectively.

Answer 7

Hydrogen has no neutron as long as you are not talking about isotopes Deuterium and Tritium..

Answer 8

oh no isotopes is in next module, so mass number need not be an integer is it ?

Answer 9

For most elements it isn't an integer.

Answer 10

It's because the mass of the neutron is not quite equal to the mass of the proton (it's heavier, which is why, when free, it decays into a proton with a half life of 14 minutes), and Carbon 12 has an equal number of protons as neutrons, so the average is taken. The masses of the electrons is so much smaller than the proton-neutron difference that it's negligible. Historically also, I'd imagine, Carbon's easier to make.

Answer 11

thnk u :)