I apologize for any confusion caused by my previous response. To clarify, the carbon-12 atom is not made up of the relative atomic mass of other atoms. The relative atomic mass of carbon-12 is exactly defined as 12 atomic mass units (u) because it is used as the standard for atomic mass.
In the past, scientists used oxygen-16 as the reference isotope with an atomic mass of 16 u. However, with the advancement of technology and more accurate measurements, it was discovered that the masses of atoms were not whole numbers but had decimal values. To avoid confusion and establish a consistent standard, carbon-12 was chosen as the reference isotope with a defined atomic mass of exactly 12 u.
The relative atomic masses of other elements are determined by comparing the masses of their isotopes to the carbon-12 standard. For example, the atomic mass of oxygen is approximately 16, which means that the average mass of oxygen atoms is about 16 times that of carbon-12.
It's important to note that the relative atomic mass listed on the periodic table for an element is an average value because it takes into account the natural abundance of the different isotopes of that element. The atomic mass may vary slightly depending on the specific isotopic composition of a sample.