Question

Why amu is defined as 1/12 of c12 isotope and not 12 grams if c12?

Answer 1

Dear Student,

An atomic mass unit or amu is one twelfth of the mass of an unbound atom of carbon-12. It is a unit of mass used to express atomic masses and molecular masses. and the mass of carbon-12 Isotope was most precisely known at that time by mass spectrometry. So this was taken as a standard mass. And the atom of this element has 12 atoms so this was assigned the mass 12 amu. Also he hydrogen was the lightest element known, so hydrogen was assigned 1 amu. So, 1 amu is equal to the 1/12 of Carbon-12 Isotope.

One atomic mass unit = 1/12 mass of c-12 atom

this means 1u = 1/12u of c-12 atom - - - -Hydrogen ( atomic mass = 1u)

so, 32u = 1/12u of c-12 X 32.- - - - -Sulphur ( atomic mass = 32u)

Regards,