[jessiemariadass21]

How to explain the relative atomic mass formula's denominator that is 1/12 of an atom of Carbon-12.
As usual instead of saying to make it 1 is there any other more reliable explanation.
Thank you.

[Borek]

In many ways it is easy to use and convenient to use, but the only viable answer is that the choice was arbitrary. Many other numbers would be as good as this one.

It is a bit like asking why the meter has the length it has, or why the second lasts exactly as long as it does. In each case the only answer is "because we decided it to be so".

Note, that these decisions were necessary to establish some reference that we can all use. So while the decisions itself were arbitrary, making them was a necessity.

[Enthalpy]

If I grasp the question properly:

it's because an atom of carbon-12 has 12 nucleons, 6 protons plus 6 neutrons (and 6 electrons). So the atomic mass unit is the mass of one nucleon.

Some luck helped here, as the mass of a neutron resembles the mass of a proton plus an electron. So in a neutral atom, protons+electrons count as much as neutrons, in a first approximation.

The ancestors could have chosen 1/1 the mass of a hydrogen atom, or 1/16 of an oxygen atom, or 1/59 of a cobalt atom, 1/232 of a thorium atom... Choosing an element that consists essentially of a single isotope makes sense. Then, as nucleons don't weigh exactly as much in all atoms, the choice of a reference has consequences. Carbon simplifies organic chemistry, and is somewhere between heavier nucleons in hydrogen and lighter ones in iron.