Neutral hydrogen is a normal, electrically
neutral hydrogen atom with one
proton and one
electron. It is commonly referred to as
HI (pronounced H-one), and is located throughout
galaxies as
HI clouds or external to
galaxies as part of the
intercloud gas . It is detected via the
spin-flip transition at 21cm in the radio, and
HI clouds were used to determine the structure of our
Galaxy from our location within it.