Atoms and molecules are electrically neutral in that the number of negatively charged electrons is exactly equal to the number of positively charged protons. Much of the "normal matter" that we find around us is in this form. However, particularly when there are energy sources available, atoms or molecules can gain or lose electrons and acquire a net electrical charge. This process is called ionization.
There is a standard notation in astrophysics for various levels of ionization of an atom. As illustrated in the following table, this notation uses increasing Roman numerals to indicate higher levels of ionization.
|Notation for Degrees of Ionization|
|I||Not ionized (neutral)||H I, He I||H, He|
|II||Singly ionized||H II, He II||H+, He+|
|III||Doubly ionized||He III, O III||He++, O++|
We also show the standard chemist's notation for such ions, which consists of placing a right superscript on the element symbol indicating the net electrical charge on the ion (in the neutral case the superscript "0" is usually omitted). Thus, for example, we shall speak later of "H II regions", meaning volumes of space in which the radiation from nearby hot stars has completely ionized the hydrogen.