The atomic mass of hydrogen is not equal to the mass of its one electron. The atomic mass of hydrogen is determined by considering the combined mass of its nucleus (which consists of one proton) and its one electron.
The atomic mass of hydrogen is approximately 1.00784 unified atomic mass units (u). This value is slightly greater than the mass of its one proton, which is approximately 1.00728 u. The additional mass in hydrogen comes from its one electron.
The mass of an electron is about 0.00054858 u, which is significantly smaller compared to the masses of the proton and neutron. Therefore, the contribution of the electron's mass to the total atomic mass of hydrogen is relatively small.
It's important to note that the atomic mass listed for an element is an average value that takes into account the different isotopes of that element and their relative abundances. For hydrogen, the atomic mass considers the isotopes protium (1H), deuterium (2H), and tritium (3H), which have different numbers of neutrons in their nuclei. The majority of naturally occurring hydrogen is protium, which contributes significantly to the average atomic mass.