The atomic weight of an atom is the ratio of an atom's mass compared to which reference atom?

The atomic weight of an atom is typically expressed relative to the carbon-12 isotope, which is specifically chosen as a standard reference. By defining carbon-12 as having an atomic weight of exactly 12 atomic mass units (amu), it allows for the comparison of the masses of all other atoms on a consistent scale.

The choice of carbon as a reference atom is due to its abundance in organic compounds, making it a fundamental building block of life. As a result, when calculating atomic weights, scientists use the relative mass of an atom compared to this standard of carbon-12, allowing for a clear and standardized set of measurements across different elements.

In contrast, hydrogen, oxygen, and nitrogen, while they may have significant roles in chemistry and biochemistry, are not used as the standard reference for atomic weight comparisons like carbon-12 is. Thus, carbon serves as the cornerstone value in the context of atomic weights.