Single particles (atoms, molecules, and ions) in the interstellar medium. Interstellar gas particles travel at considerably less than the speed of light, in contrast to galactic cosmic rays. They may have a charge due to some loss of electrons.
Atoms that have the same number of protons but a different number of neutrons. They are atoms of the same element that have different masses. The isotope number is the number of protons plus the number of neutrons.