The atomic mass of hydrogen is not rounded to one because the atomic mass is a weighted average of the masses of its isotopes. Hydrogen has three naturally occurring isotopes: hydrogen-1 (or protium), deuterium (hydrogen-2), and tritium (hydrogen-3).
The most common isotope of hydrogen is hydrogen-1, which has a mass of approximately 1 atomic mass unit (amu). However, deuterium and tritium have different masses due to the presence of additional neutrons. Deuterium has a mass of approximately 2 amu, and tritium has a mass of approximately 3 amu.
To calculate the atomic mass of hydrogen, the masses of the three isotopes are taken into account, along with their relative abundances in nature. Since hydrogen-1 is the most abundant isotope, its contribution to the atomic mass is the most significant. However, the contributions from deuterium and tritium, although relatively small, still have an impact on the overall atomic mass.
Therefore, when the atomic masses of the isotopes are weighted according to their abundances, the resulting atomic mass of hydrogen is approximately 1.008 amu. This value is not rounded to one because it takes into account the contributions from all three isotopes of hydrogen.