Final answer:
The most abundant isotope of hydrogen is Protium (¹H) because its mass of 1 amu is closest to hydrogen's average atomic mass of 1.008 amu. Over 99.98% of naturally occurring hydrogen is Protium, making it far more prevalent than the other isotopes.
Step-by-step explanation:
The isotope of hydrogen that is most abundant is Protium (¹H) because it has an atomic mass closest to the average atomic mass of hydrogen, which is 1.008 amu. Protium has one proton and no neutrons, and it is significantly more abundant in nature than the other isotopes. Deuterium (²H) has one proton and one neutron, and Tritium (³H) has one proton and two neutrons. Due to their higher atomic masses of 2 and 3 amu respectively, and the average atomic mass of hydrogen being so close to 1 amu, Deuterium and Tritium are much less common.
Naturally occurring hydrogen is composed of these three isotopes, but over 99.98% of it is Protium, indicating it is by far the most prevalent. This is also supported by comparing the masses of the isotopes to the weighted average as found on the periodic table, where Protium's mass of 1 amu is most consistent with the weighted average atomic mass of hydrogen.