# What atom does not have any neutrons?

The protium ""^1H isotope.
Most atoms in this universe are this isotope. The nucleus of the ""^1H isotope contains a single, massive, positively charged particle, and hence its mass $\equiv$ $\text{1 amu}$.
Of course, deuterium, ""^2H, and tritium, ""^3H, isotopes are also known. How many neutrons are found in these atoms?