Question

Question #6ab6e

Answer 1

Atomic Masses are Relative - they are not absolute.

Explanation:

1 amu is defined as being 1/12 of the mass of an atom of carbon-12.

A proton and a neutron do not actually weigh 1amu, they weigh slightly more but their mass / energy changes when they reside in an atoms nucleus (this is called 'binding energy') and so we describe them as having a mass as 1/12 of the mass of an atom of carbon-12.

From this, if we then 'know' from chemical analysis and experimentation that an atom has 7 protons and 7 neutrons, we can describe it as having a relative atomic mass of 14 (7/12 + 7/12).

Carbon doesn't HAVE to be used for this process, theoretically any element bigger or smaller than the lightest and heaviest elements could be used. Infant, in the first half of the 20th Century it was Oxygen that was used as the reference atom for relative atomic mass. This was changed to carbon because of the abundances of isotopes of oxygen. But the science world settled on Carbon and so that is why carbon-12 is used.

The important thing to remember is that it is all relative though. Rather like a metre in length is actually defined as 'the distance travelled by light in 1/299792458' of a second. We can only measure things relative to one another!