* Atomic mass is primarily determined by the number of protons and neutrons in an atom's nucleus.
* Hydrogen (H) has an atomic mass of approximately 1 because it has 1 proton and no neutrons.
* Helium (He) has an atomic mass of approximately 4 because it has 2 protons and 2 neutrons.
While there are isotopes (atoms of the same element with different numbers of neutrons), the most common isotope of Helium has 2 protons and 2 neutrons, giving it an atomic mass of 4.
It's possible you're thinking of tritium, a rare isotope of hydrogen. Tritium has 1 proton and 2 neutrons, giving it an atomic mass of approximately 3. However, tritium is not a naturally occurring atom, but rather a radioactive isotope produced artificially.
