What do you mean by the atomic mass of an element? Which carbon isotope is used for atomic mass reference and why?

Atomic mass: The mass linked to an atomic particle is referred to as its atomic mass. One atomic mass unit is equal to exactly one-twelfth ($\frac{1}{{12}^{\mathrm{th}}}$) mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12. Atomic masses of elements are expressed in 'U’ (or) a.m.u, 'U’ means unified mass.

We cannot measure atomic mass accurately and directly, so we have to use a reference. Even though we have many references, nowadays, we consider carbon-12 as the reference as it gives an accurate atomic weight.

The formula of atomic mass is-

Atomic mass = $\frac{\mathrm{Mass} \mathrm{of} \mathrm{one} \mathrm{atom} \mathrm{of} \mathrm{an} \mathrm{element}}{{\displaystyle \frac{1}{{12}^{\mathrm{th}}}}\mathrm{part} \mathrm{of} \mathrm{mass} \mathrm{of} \mathrm{an} \mathrm{atom} \mathrm{of} \mathrm{C}‐12 \mathrm{isotope}}$

The atomic mass of a few elements are:

Hydrogen: 1 U

Carbon: 12 U

Nitrogen: 14 U