Atomic mass of hydrogen?
The mass number is the number of protons added to the number of neutrons.
Hydrogen only contains 1 proton and no neutrons, so that gives a of 1.
However, there are some isotopes of Hydrogen, in other words, some atoms that have more than zero neutrons. For example, Deuterium has 1 neutrons hence an atomic mass of 2 and Tritium has 2 neutrons hence an atomic mass of 3.
The number you see for atomic mass in a periodic table is an average of the abundance of the isotopes, which is why it’s a decimal like ##1.00794## and not a whole number.