What is the most common isotope of hydrogen?

Answer 1

The most common isotope of hydrogen is hydrogen-1, designated ¹H. It has one proton in its nucleus, and one electron (gotta keep the charges balanced).

Hydrogen-2, designated ²H (and sometimes ²D, is called Deuterium, and is also called heavy hydrogen. It has one proton and one neutron (if you change the number of protons, it wouldn't be hydrogen) and one electron.

Hydrogen-3,designated ³H and also called Tritium, has one proton and two neutrons, and one electron. It is radioactive.

Answer 2

H-1 (¹H) is the most common isotope of hydrogen.