Question

So since 1 atomic unit is exactly 1/12 the mass of 1 c-12 atom,
can we say as follows;
atomic mass of sodium is 23 , so can we say that the atomic mass of sodium is 23 times heavier than 1/12 the mass of 1 carbon-12 atom?

Answer 1

Initially, when the first standards were set, the calculation of molecular mass was indeed based on the Hydrogen-1 element. But, later the norms and standards were shift to the Carbon C-12 isotope.
The reason is pretty simple and logical. The mass of the Hydrogen-1 element is about 1.007825 u."u" being the atomic mass unit. Whereas the mass of Carbon C-12 isotope is almost exactly 12.oooo u. So the 1/12 the mass of carbon -12 isotope will be exactly 1 u. But, that won't be the case for Hydrogen-1 as its mass is not exactly 1 u but 1.007825 u. The decimal part may seem insignificant but, it does result into huge errors for heavier element and hefty calculations. So, in order to standardize the result and make them more accurate the IUPAC decided to make the above mentioned change.
atomic mass of sodium is 23 , so can we say that the atomic mass of sodium is 23 times heavier than 1/12 the mass of 1 carbon-12 atom