This is a very complex question that depends where you are standing.
Near the Sun, we are located in what is called the Local Bubble (above sketch). It extends about 100 - 300 parsecs from the Sun and is a region in which the little interstellar matter that is present at densities of 0.01 atoms/cc is virtually all in the form of ionized hydrogen. No one has found any atomic hydrogen or molecular hydrogen in this peculiar region.
The interstellar medium contains clouds, and an 'intercloud' medium which itself may have 2 or three different components. The neutral intercloud medium has a density of about 1 atom of hydrogen per cubic centimeter. Slightly higher within the spiral arms of the Milky; slightly lower between the arms. This 'HI' medium is easily mapped at 21-centimeters using a radio telescope, and consists of clouds many light years across with densities slightly higher than 1 atom/cc.
There also seems to be a 'warm interstellar medium' consisting of a plasma of ionized hydrogen and some other detectable elements such as calcium and sodium. This medium is probably created by supernovae remnants of hot plasma that eventually intermingle with the rest of the interstellar medium over the course of millions of years.
Finally, these is a cloudy component where gas collects into light year-sized, dense clouds in which stars may form. These clouds have a very complex chemistry. At their surfaces where the density is near 1 - 10 atoms per cc, some of the gas may be ionized hydrogen if a nearby luminous star is present capable of ionizing the gas. This occurs in the Orion Nebula. Deeper into the cloud, the ionized component vanishes because the radiation cannot penetrate farther. The gas here is in atomic hydrogen form. Still farther in, at densities of 100 - 1000 atoms/cc, hydrogen molecules begin to dominate over the atomic form and you enter a regime in which a rich chemistry can occur producing many species of molecules such as carbon monoxide, formaldehyde, ammonia, and various alcohols.
Copyright 1997 Dr. Sten Odenwald
Return to Ask the Astronomer.