The atomic mass of hydrogen is 1 and not 2 because the atomic mass of an element is determined by the average mass of all its naturally occurring isotopes, taking into account their relative abundance.
Hydrogen has three naturally occurring isotopes: hydrogen-1 (protium), hydrogen-2 (deuterium), and hydrogen-3 (tritium). The vast majority of hydrogen atoms (more than 99.9%) are hydrogen-1, which consists of a single proton and no neutrons. Hydrogen-2, also known as deuterium, has one proton and one neutron, making it twice as heavy as hydrogen-1. However, deuterium is relatively rare, accounting for only a small fraction of naturally occurring hydrogen.
When calculating the atomic mass, the relative abundance of each isotope is taken into consideration. Since hydrogen-1 is the most abundant isotope, its contribution dominates the calculation. The atomic mass of hydrogen is, therefore, closer to the mass of hydrogen-1, which is approximately 1 atomic mass unit (u), rather than the heavier hydrogen-2 isotope.
It's worth noting that the atomic mass of an element is not necessarily a whole number because it takes into account the weighted average of the masses of its isotopes, which may have fractional atomic masses due to the presence of neutrons.