The mass of an atom compared to the mass of a carbon-12 atom is called the atomic mass unit (amu).

Here's why:

* Carbon-12 as a Standard: Carbon-12 is the standard for measuring atomic mass. It has been assigned a mass of exactly 12 atomic mass units (amu).

* Relative Mass: The atomic mass of all other elements is determined relative to the mass of carbon-12.

* amu Definition: One atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 atom.

So, when you see an element's atomic mass listed on the periodic table, it's telling you how much heavier or lighter that element is compared to a carbon-12 atom.