The relative atomic mass of hydrogen is not equal to the relative atomic mass of a proton plus the relative atomic mass of an electron. The reason for this is that the relative atomic mass of an element takes into account the average mass of all isotopes of that element and their respective abundances.
The atomic mass of hydrogen is primarily determined by the most common isotope, hydrogen-1 (1H), also known as protium. It consists of a single proton and no neutrons. The mass of a proton is approximately 1.00727647 atomic mass units (amu), while the mass of an electron is much smaller and negligible in comparison.
The exact relative atomic mass of hydrogen is defined as 1 atomic mass unit (amu) since it is used as the reference for atomic masses. This means that the mass of hydrogen-1 is approximately 1 amu. The calculation involves a weighted average of the masses of the different isotopes of hydrogen and their respective abundances. However, since the abundance of hydrogen-1 is nearly 100%, its contribution dominates the average, resulting in a value close to 1 amu.
It's important to note that the atomic mass of hydrogen does not equal the sum of the masses of its constituent particles (proton and electron), as the mass of an atom also includes the contribution from the binding energy of the nucleus.